Falling Sand glitches through block

A piston is hooked up to a button and also to a monostable circuit. A sand block is placed on top of a another block, which is placed on a sticky piston.
First the piston extends and pushes both blocks up. 12 ticks after activating the button the monostable circuit gets activated and the piston (already retracted) receives a 1 tick pulse.
The bottom block is pushed up and stays there while the piston retracts. Then the sand block falls right through the other block and lands on the piston.
I also made a video and attached it.