When a piston or sticky piston is powered by a redstone block, it extends, then, when a redstone block is placed above the block above the piston or sticky piston, and both redstone blocks are removed, only until the piston receives a block update such as a block bordering it being placed or broken, does it retract.

What I expected to happen was...:
I expected that when the the piston's power blocks were broken, that the piston or sticky piston would retract.

What actually happened was...:
The piston/sticky piston stayed extended; even the debug screen (F3) displayed the piston as extended.

Steps to Reproduce:
1. Power a sticky piston or piston with a redstone block.
2. Place a redstone block two blocks above the said piston or sticky piston.
3. Break the redstone blocks; first the one bordering the said piston or sticky piston, and then the one above the block above the said piston or sticky piston.
4. Update the piston block or the piston head by breaking or placing a block bordering the piston block or piston head, which will retract the piston/sticky piston.