The hitbox of the piston body is too large when it is extended. The hitbox of the piston head matches its shape, but the hitbox of the piston body is a full block and doesn’t match its shape. This prevents you from interacting with blocks behind the piston. For the piston head the hitbox also used to be a full block, but that changed in 1.8, but the piston body still has the wrong hitbox. On java edition that part of the block is completely missing from the hitbox MC-124459.