Here's the same output in R&D (which creates the tuns correctly):
write(2, "20171213204857407 DGGSN <0002> g"..., 6820171213204857407 DGGSN <0002> ggsn.c:201 APN(inet6): Starting
ESC[0;m) = 68
write(2, "20171213204857408 DGGSN <0002> g"..., 8320171213204857408 DGGSN <0002> ggsn.c:204 APN(inet6): Opening TUN device tun6
ESC[0;m) = 83
open("/dev/net/tun", O_RDWR) = 6
ioctl(6, TUNSETIFF, 0x7fff7c1ba790) = 0
ioctl(6, TUNSETNOCSUM, 0x1) = 0
write(2, "20171213204857411 DGGSN <0002> g"..., 8220171213204857411 DGGSN <0002> ggsn.c:209 APN(inet6): Opened TUN device tun6
ESC[0;m) = 82
fcntl(6, F_GETFL) = 0x8002 (flags O_RDWR|O_LARGEFILE)
fcntl(6, F_SETFL, O_RDWR|O_NONBLOCK|O_LARGEFILE) = 0
fcntl(6, F_GETFD) = 0
fcntl(6, F_SETFD, FD_CLOEXEC) = 0
write(2, "20171213204857412 DGGSN <0002> g"..., 10920171213204857412 DGGSN <0002> ggsn.c:232 APN(inet6): Setting tun IPv6 address fde4:8dba:82e1:2000::/56
ESC[0;m) = 109
access("/proc/net", R_OK) = 0
access("/proc/net/unix", R_OK) = 0
socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 7
ioctl(7, SIOCGIFINDEX, {ifr_name="tun6", }) = 0
close(7) = 0
socket(AF_INET6, SOCK_DGRAM, IPPROTO_IP) = 7
ioctl(7, SIOCSIFADDR, {ifr_name="\375\344\215\272\202\341 ", ifr_addr={sa_family=0x38 /* AF_??? */, sa_data="\0\0\272\1\0\0\1\0\0\0\0\0\0\0"}}) = 0
close(7) = 0
socket(AF_INET, SOCK_DGRAM, IPPROTO_IP) = 7
ioctl(7, SIOCSIFFLAGS, {ifr_name="tun6", ifr_flags=IFF_UP|IFF_RUNNING}) = 0
close(7) = 0
socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE) = 7
bind(7, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, 12) = 0
getsockname(7, {sa_family=AF_NETLINK, nl_pid=25384, nl_groups=00000000}, [12]) = 0
sendto(7, {{len=20, type=0x12 /* NLMSG_??? */, flags=NLM_F_REQUEST|0x300, seq=1513194537, pid=0}, "\0\0\0\0"}, 20, 0, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, 12) = 20
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=1232, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\4\3\1\0\0\0I\0\1\0\0\0\0\0\7\0\3\0lo\0\0\10\0\r\0\1\0\0\0"...}, {{len=1240, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0\2\0\0\0C\20\1\0\0\0\0\0\t\0\3\0eth0\0\0\0\0\10\0\r\0"...}, {{len=1240, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0\3\0\0\0C\20\1\0\0\0\0\0\t\0\3\0eth1\0\0\0\0\10\0\r\0"...}, {{len=3352834104, type=0x7fdd /* NLMSG_??? */, flags=0, seq=3374275523, pid=32733}, "\334\5\0\0\0\0\0\00080\330\307\335\177\0\0\260z?\311\335\177\0\0\310\244\33|\377\177\0\0"...}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 3712
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=1240, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0\4\0\0\0\3\20\0\0\0\0\0\0\t\0\3\0eth2\0\0\0\0\10\0\r\0"...}, {{len=1236, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0\5\0\0\0\2\20\0\0\0\0\0\0\n\0\3\0wlan0\0\0\0\10\0\r\0"...}, {{len=1244, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0Q\0\0\0\2\20\0\0\0\0\0\0\22\0\3\0wwp0s18f2u1u"...}, {{len=3374275523, type=0x7fdd /* NLMSG_??? */, flags=0, seq=1500, pid=0}, "80\330\307\335\177\0\0\260z?\311\335\177\0\0\310\244\33|\377\177\0\0\304\244\33|\377\177\0\0"...}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 3720
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=1248, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0R\0\0\0\2\20\0\0\0\0\0\0\24\0\3\0wwp0s18f2u1u"...}, {{len=1220, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\376\377S\0\0\0\220\20\0\0\0\0\0\0\24\0\3\0wwp0s18f2u1u"...}, {{len=1236, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0T\0\0\0\2\20\0\0\0\0\0\0\n\0\3\0wwan1\0\0\0\10\0\r\0"...}, {{len=0, type=0 /* NLMSG_??? */, flags=0, seq=524293, pid=0}}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 3704
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=1224, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\376\377U\0\0\0\220\20\0\0\0\0\0\0\24\0\3\0wwp0s18f2u1u"...}, {{len=1236, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\1\0V\0\0\0\2\20\0\0\0\0\0\0\n\0\3\0wwan2\0\0\0\10\0\r\0"...}, {{len=1228, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\376\377\271\1\0\0Q\0\1\0\0\0\0\0\t\0\3\0tun4\0\0\0\0\10\0\r\0"...}, {{len=0, type=0 /* NLMSG_??? */, flags=0, seq=524293, pid=0}}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 3688
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=1228, type=0x10 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\376\377\272\1\0\0Q\0\1\0\0\0\0\0\t\0\3\0tun6\0\0\0\0\10\0\r\0"...}, {{len=131088, type=0x8429 /* NLMSG_??? */, flags=NLM_F_REQUEST|NLM_F_DUMP_INTR|NLM_F_DUMP_FILTERED|0x5a00, seq=25384, pid=65536}, "V\0\0\0\2\20\0\0\0\0\0\0\n\0\3\0wwan2\0\0\0\10\0\r\0\350\3\0\0"...}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 1228
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=20, type=NLMSG_DONE, flags=NLM_F_MULTI, seq=1513194537, pid=25384}, "\0\0\0\0"}, {{len=442, type=0x51 /* NLMSG_??? */, flags=NLM_F_REQUEST, seq=0, pid=196617}, "tun6\0\0\0\0\10\0\r\0\364\1\0\0\5\0\20\0\0\0\0\0\5\0\21\0\0\0\0\0"...}, {{len=0, type=0 /* NLMSG_??? */, flags=0, seq=0, pid=0}}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 20
sendto(7, {{len=20, type=0x16 /* NLMSG_??? */, flags=NLM_F_REQUEST|0x300, seq=1513194538, pid=0}, "\0\0\0\0"}, 20, 0, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, 12) = 20
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=76, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\10\200\376\1\0\0\0\10\0\1\0\177\0\0\1\10\0\2\0\177\0\0\1\7\0\3\0lo\0\0"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\200\0\2\0\0\0\10\0\1\0\n\t\31e\10\0\2\0\n\t\31e\10\0\4\0\n\t\31\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\200\0\3\0\0\0\10\0\1\0\n**\1\10\0\2\0\n**\1\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\2\10\0\2\0\n**\2\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\3\10\0\2\0\n**\3\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\4\10\0\2\0\n**\4\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\5\10\0\2\0\n**\5\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\6\10\0\2\0\n**\6\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\7\10\0\2\0\n**\7\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\10\10\0\2\0\n**\10\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**\t\10\0\2\0\n**\t\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**2\10\0\2\0\n**2\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\201\0\3\0\0\0\10\0\1\0\n**3\10\0\2\0\n**3\10\0\4\0\n**\377"...}, {{len=88, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\200\0\4\0\0\0\10\0\1\0\n@\1\1\10\0\2\0\n@\1\1\10\0\4\0\n@\1\377"...}, {{len=80, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\200\0\4\0\0\0\10\0\1\0\300\250\1\310\10\0\2\0\300\250\1\310\t\0\3\0eth2"...}, {{len=80, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\2\30\200\0\271\1\0\0\10\0\1\0\260\20\336\1\10\0\2\0\260\20\336\1\t\0\3\0tun4"...}, {{len=0, type=0x24 /* NLMSG_??? */, flags=NLM_F_MULTI|NLM_F_ACK|NLM_F_ECHO, seq=0, pid=0}}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 1380
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=72, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\n\200\200\376\1\0\0\0\24\0\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\1\24\0\6\0"...}, {{len=72, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\n@\200\375\2\0\0\0\24\0\1\0\376\200\0\0\0\0\0\0\2\r\271\377\3765:@\24\0\6\0"...}, {{len=72, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\n@\200\375\3\0\0\0\24\0\1\0\376\200\0\0\0\0\0\0\2\r\271\377\3765:A\24\0\6\0"...}, {{len=72, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\n@\200\375\271\1\0\0\24\0\1\0\376\200\0\0\0\0\0\0\216l\342-s>\364\265\24\0\6\0"...}, {{len=72, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\n8\200\0\272\1\0\0\24\0\1\0\375\344\215\272\202\341 \0\0\0\0\0\0\0\0\0\24\0\6\0"...}, {{len=72, type=0x14 /* NLMSG_??? */, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\n@\200\375\272\1\0\0\24\0\1\0\376\200\0\0\0\0\0\0A@B1l\317\247\232\24\0\6\0"...}, {{len=131092, type=0x842a /* NLMSG_??? */, flags=NLM_F_REQUEST|NLM_F_DUMP_INTR|NLM_F_DUMP_FILTERED|0x5a00, seq=25384, pid=8460290}, "\3\0\0\0\10\0\1\0\n**\4\10\0\2\0\n**\4\10\0\4\0\n**\377\v\0\3\0"...}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 432
recvmsg(7, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=20, type=NLMSG_DONE, flags=NLM_F_MULTI, seq=1513194538, pid=25384}, "\0\0\0\0"}, {{len=1, type=0x14 /* NLMSG_??? */, flags=NLM_F_REQUEST, seq=0, pid=0}}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 20
close(7) = 0
I see no big differences. I attach the full strace log.