When a block affected by gravity (sand & gravel) falls on a piston of any type the physics block will break regardless of the side of the piston is struck from, similar to what happens when a physics block falls on a cactus.
The standard interaction between these blocks should be that the physics block remain in block form once it lands on the piston, this is how the blocks interact in the java based version of the game.
blocks affected: Sand, gravel, pistons, sticky pistons
YouTube Video of the bug.
https://youtu.be/YzgD4R-6bbE