When you land on a jump-through block theres a big chance that you player can get inside of the block. Like shown on this picture.
I found out a way to fix this that is acctually pretty simple.
In the event sheet you add a condition: Player is overlapping with Jump-through block. and another Inverted one: Platform is jumping. And then add an action: Set Y to Player.Y - 3. And this should fix the problem. this also fixes the one where if you move from a normal block to a jump-through one you can just fall through.