stacked sand/gravel falling after being pushed up by a double piston extend will break one

When you place 2 or more sand/gravel on top of a double piston extender, then expand and retract it one of the sand/gravel will be destroyed if both pistons are sticky pistons.
I expected all the sand to just fall back down, but the sticky piston pulled one sand and the other fell into the same block.

To recreate:
1. Build a double piston extender with 2 sticky pistons like the one in the attachments.
2. Place 2 or more sand/gravel on top of the pistons.
3. Power it on, then power it off.