Artpunk, When remaking the Jump-thru behavior from scratch, one would have to check, for each solid object, if the player at a Y of -1 is overlapping the solid object. If not, enable collisions with that object.
In this case, one could simply use this check to, rather than enable/disable collisions, add if player is falling, and if so, set vector Y to 0.
The fact that this is happening, though, points to a problem with how you've implemented controls, positioning, collision masks, etc. It could be anything, but this kind of functionality doesn't happen naturally with platform and jump-thru behaviors, it certainly isn't a bug as I've never seen this happen before.
EDIT: make sure that when you use the event I mentioned that you're also adding a condition of player is overlapping the platform at y offset of +1, or the player won't be able to fall.