[Splash]

This could be a possible update
Portal Escape
If any figure in the Upside Down ends its movement on an Exit Space or if at the end of a round any figure without The Upside Down special power is in the Upside Down battlefield, the player that controls that figure must place it on any empty space on any other battlefield in this game.

They'd only being taking a possible max of three wounds from being pulled there though, with an average of like one maybe. With the timer we could change unblockable to just "receives a wound". So, it keeps a reason to use the powers involved and the design healthy.

Or are we thinking more this (no exit space?):
Portal Escape
If at the end of a round any figure without The Upside Down special power is in the Upside Down battlefield, the player that controls that figure must place it on any empty space on any other battlefield in this game.