This happens to me multiple times when I'm trying to join a co-op match. During my first ever Beta 2 co-op matchmaking it still found me a match where I can play but after a couple of games it kept putting me in the spectators.

I noticed that also and it won't let you choose your class or layout. Even after the round has ended (if your team has been eliminated) it still won't let you join them and keeps you in Spectator mode.

This happens to me constantly, at least over 60% of the time when I try to join coop,