In 4.27 the logic in SteamSocket.cpp which is part of the SteamSockets plugin was changed to use Poll Groups instead of a removed API function that checked all children of a listen socket. However, FSteamSocket::Accept does not actually assign the accepted child connections to the poll group so if you use steam sockets to create a listen server the call to ReceiveMessagesOnPollGroup does nothing. This was discovered from licensee reports, but this should reproduce in shootergame if steamsockets are enabled.
One possible fix is to set it up so it only creates a new poll group in Listen(), and then initializing the child sockets to use the same poll group as the original listen socket.
This repro is theoretical and unconfirmed, the issue was found on a licensee project
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-132744 in the post.