This has been a problem for months and is consistently reproducible. If you create a co-op session with Epic Games players (game master is on Steam in my case, but not sure if that matters), then everyone except the game master will disconnect after 60-80 mins. This happens every time. Every player needs to restart their game fully and create a new session.
I figured this would've been fixed in the patch today, but it wasn't. Is this ever going to be fixed? It's such a massive problem.