Its not a problem, its not the way I would normally use it. I've got two accounts and players have an account. But it was nice and quick so I did it thinking it would work.
I assumed rejoin as player meant you were in as an actual dummy player, not as the GM. But I just tried it again and yes, I'm under the same user so that probably where the problem is.
/gmroll should go to the players at all though, only to the GM. In this instance its confusing since there are two GM users and one just doesn't have GM rights.
I don't think it needs to be fixed. But I noticed it right away when the players could see and hear the roll I was doing. :)
One bonus is that the measuring tool doesn't show up on the "Player's" window when the GM is measuring something. But if you are logged in as a separate player then it does show up. I've got a bug report about it complete with screen capture showing it.