After selecting a class, you have to wait in Spectator until the start of the next round. You don't instantly spawn. It's like Counter Strike. Short rounds with limited lives, a "last man standing" kinda deal. Like Fortnite! Kinda. So mid-round joining isn't a thing. (Except in Duel mode.)
That being said, sometimes (especially when selecting Jedi/Sith) you'll get "join-bugged", and you won't spawn. If that happens, I think you have to restart your game? Or maybe just rejoin the server. Maybe restart your game. Not sure.