I´ve looked at it and yes, it´s a shifting collision polygon. It´s the animation that causes one part of the enemy to poke below the platform and the engine thinks it dropped through. A simple fix is to set it at least semi-consistent by making it look like this and use "apply to whole animation"
The same goes for the player btw. but he seems fine, probably just by chance. Ideally you would want to use an invisible sprite for the hitbox that is consistent and have the visible sprite pinned to it.