Mt7622 hardware flow offload bug?

Hi, just a small question. I'm having recurring problems where my OpenWRT firewall loses a UDP conntrack between an IPv6 host inside my network and one outside. Firewalling is pretty straightforward.

What happens is that conntrack -E -o timestamp -p udp -f ipv6 does not show a connection between a host inside the network and outside. There is however an older conntrack entry with a different port. I.e. the one that is there:
udp 17 src=2a02:XXXX:13eb dst=2a01:XXXX:14a2 sport=40678 dport=37625 packets=297355 bytes=70650880 src=2a01:XXXX:14a2 dst=2a02:XXXX:13eb sport=37625 dport=40678 packets=487724 bytes=97575028 [HW_OFFLOAD] mark=0 use=2

The one the machine tries to setup is all the same, but coming from UDP 50021.

Now I was going to send a message to the netfilter mailing list but I'm actually suspecting the "HW_OFFLOAD" part of the visible connection. Is there a way to test this? (Apart from turning off the HW Offload, as I just did, and wait a few weeks?)