When pistons are facing up and at Y=254, they will not extend until an immovable block that is broken by pistons is placed on it.
Steps to reproduce:
1. Place a piston at Y=254.
2. Place a block next to the piston.
3. Place a lever on the block.
4 . Activate the lever.
Expected results: The piston will extend normally.
Actual results: The piston will not extend until an immovable block (buttons, levers, redstone dust etc.) that a piston can break is placed on it.