I'm not sure what's the exact problem. It's something with redstone, block update, and piston direction. I attached two screenshots which will explain the problem.
Basically, when I power up a piston looking down, with a block and redstone dust on top with a power of 12 (or less), and the turn off the power, the piston will stay extended and wont retract.
If the piston is looking in any other direction, it does update (in that construction, perhaps because it has a block and redstone dust to the side)
If I put a block next to the piston while extended (when powered off), it updates.