Are you seeing this when using UEFN to launch the session? If so, can you search your Output Log after enabling LogMatchmakingServiceClient (Filters → Filters → click the checkbox by LogMatchmakingServiceClient) for something along the lines of
LogMatchmakingServiceClient: Verbose: HandleWebSocketMessage - Received message:
with reference to a sessionId later in the log message? Would you mind DM’ing me this log message if you’re able to find it (I really only need the value corresponding to sessionId)? If the issue only occurs when running outside of UEFN, that would also be useful information.