On beta had similar issue, sometimes if you log in on another character(without lobby function), then go back to desired one would solve, try to do it without reentering in the same game as before, if works you should be able to log on the older room if the issue weren’t the room itself.
When that kind of event happens sometimes the room gets “locked” not enabling you to rejoin in the same game.
If doesn’t solve I would recommend keep updated on blizzard cs, forums and maybe submit a ticket.