Hyper-V hn NICs drop UDP6 traffic when transmit checksums are enabled
Due to a problem with FreeBSD 11.2, Hyper-V NICs can't send IPv6 UDP traffic when transmit checksums are enabled.
See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231797 for more details and a patch.
No easy workaround since disabling checksums also has an issue with IPv6 transmit checksums, see #8980
User can manually run
ifconfig hn0 -txcsum6 as a temporary workaround.