Hello,
I have a similar issue: https://forum.free5gc.org/t/upf-not-forwarding-packets-received-in-gtp-tunnel-with-ueransim/1263 . I received an answer indicating that the problem could be similar to the one indicated here: https://forum.free5gc.org/t/issue-with-tcp-traffic-and-nat-running-free5gc-in-openstack/1156, but I haven’t found any problem with checksum in my wireshark traces, as indicated in my thread.
I hope we could receive any additional indication helping to solve our issue.
Regards,
José María