Just tried to play a game of RCBM with @stuart98. There were 3 spectators, and the same thing happened to @ZaphodX 's stream a few days ago. I believe the same thing happens when clicking ready right before the commander screen comes up. I've made multiple threads on these lobby bugs, they're truly one of the most annoying things that can happen.
i was 1 of those spectators in that game and it buged on me too, same on with Zaphods stream i was a spectator in that too, same again bugged out with full lobby 10player ffa 3 spectators.
I thought the problem suspected to be "10 player maximum for servers" so if you have 10 people you can have 0 spectators. Bypassing 10 with spectators and players causes this freeze I believe.
I just tested spectating with ten AI with out issue. Are you sure someone didn't unready as described in: PA#4125? I've mostly only had that bug in ZaphodX matches (someone trolling him me thinks).