Falling blocks on piston head will drop as an item when the piston retracts

The bug

When a falling block lies on top of a piston head, it will drop as an item when the piston retracts. This seems to only happen when the sand is updated at the same time. Breaking the redstone block in

will result in the sand dropping as an item. However, if the redstone block was in the back of the piston, the sand would fall normally.