I have a firewall box where the outbound connection to cable is ue / axe. If txcsum is enabled, PF-NAT-ed outbound traffic has a zero checksum. Locally generated outbound traffic is fine.
Disabling txcsum resolves the issue and restores connectivity through the NAT.
Worked fine on 11.2-p9.
Currently on 12.0-RELEASE-p3.
Diffing the sources used, reveal no changes. Maybe the problem is some mbuf flags changed for TX checksum offloading?
git diff origin/releng/12.0 origin/releng/11.2 sys/dev/usb/net/if_axe.c
Sounds similar to 235607