Hello @Jimmy_Volmer ,Welcome to the forums!
From what I can see in the error you’re showing, it looks like when the client disconnects, Steam needs a few seconds to mark the lobby as available again. Even though the UpdateSession completes successfully on the host, during that brief period the session may not appear in the FindSessions results.
As a test, you could try adding a small delay before calling FindSessions on the client when you detect the disconnect. For example, using a Set Timer by Event for 2 or 3 seconds and, once that time has passed, performing the session search again.
I’m also including a post I found and some documentation that might be useful.
Documentation:
If this doesn’t solve the issue, could you show how you have the reconnection logic implemented? Seeing the code would help identify where the problem might be occurring.
Hope it helps!