While in the Cabaret Luna spawn room, walk over to the bar area to the right, jump over the counter and stand against the back wall. This will cause you to suddenly reappear on the stage as though you just spawned.
I can easily reproduce this bug every time I try to.
Update: it appears that the bug only occurs during the countdown before the spawn room doors open. Once the doors are open the bug no longer occurs. I am also unable to reproduce the bug while in a skirmish.
I’d wager that every spawnroom is surrounded by collision walls that reset you should you manage to glitch your way out before the match starts, and in this case that invisible barrier is clipping into the traversable area of the spawn room.