You die in a Skywars game when you're the second last player, then you respawn in the same game (e.g on the island where you died). Once this occurs, you can read the lobby chat, but your body is in a game and you can still build or do what you want. To fix it you just have to die again, and it has it has only started happening in the past month or so from what it seems like.
I've experienced this problem but I'm not sure what the source of it is. Hopefully staff can find out and fix it soon.