Two things:
1: I got pushed out of the map by hundreds of enemies pushing at the same time and can't get back in.
2: I'm afk on the hardest possible settings.
Viewing post in Pixel Survivors: Roguelike comments
The boss 30, Pumpking, might be the reason.
It has really outstanding pushing power and one creepy special move, which might force your character into the area out of the border.
There's a chance to be pushed back again by the boss. Or just try raising the move speed over 30. You would soon break the border by yourself.