I came across this very same issue with fika, a community-made mod for Escape from Tarkov. One player would consistently fail to join games and it took ages to figure out the different components that were failing. The code intentionally sent the join message 4 times in quick succession, which triggered the DoS protection on the internet firewall. Ok, disabled that. The next issue was the packets were being interfered with by the ALG on the internet firewall, so disabled that too. Then the last final hurdle was the Rx offloading on the Intel NIC which was the exact same issue with the checksum being set to all 0’s or all F’s.

What made it confusing at the time is the join packet would sometimes be accepted and passed through to the game, so it prompted further digging into why.