This is an old bug.
When a room is full (55 players) it dissapears from the list when trying to join it using "find" (everybody does this)
(this is really two bugs in one.
solution: allow 55 players trough the normal GUI (since most players know to use Find)
don't allow to join a full (55) room from "Find"
( you don't even have to fix the bug that removes the Room from the list.
Full room bug(s) - Bug reports
The client/server doesn't keep very accurate track of the # of players in a room. The client also doesn’t even know the room limit, it only knows the limit to turn the room red. When the client tries to join a full room, the server just ignores it, which causes the client to remove the room(default behavior for when a room is down). At this point without redoing the client/server protocol I don’t think there is much we can do about it or pm.net for that matter. It requires a coordinated type of fix.