I've found a temporary fix for it:

Spawn two opposing enemy groups to fight each other. Possess one of the characters. Your game will be glitched.

once the combat is over, switch out of the character you've possessed and into another (they must have been part of the combat).

Your game is now fixed and will carry on to whichever character you possess next

Note: the character that you have used to fix your game who was in combat MUST stay alive, deleting them or otherwise killing them reverts the game to its bugged form

Note 2: The character you possessed initially doesn't seem to need to be alive to make it work, although it often breaks if they're not. Not exactly sure on the exact cause