It could be a compatibility issue since I'm using version 1.6.2 of RPG Maker MV but I'll look further to see what the problem is.
Edit: It seems when I turn hp_OliviaVictoryAddon (add extra window to show actor level up) to ON the game bugs out when paired with the comeback system plugin set to ON and the Olivia_VictorySequenceUI plugin set to ON. Turning it OFF (the extra window to show actor level up) seems to do the trick. Could you try to replicate the issue while those three plugins are set to ON?