Sticky Pistons pushing Redstone blocks do not retract.

Steps to repro:
1. Create new map in Snapshot 13w02a or 13w01b.
2. Place sticky piston facing upward
3. Place redstone block on top of piston
4. Apply and toggle redstone signal to piston
5. Observe piston not retracting
6. Destroy redstone block
7. Observe

Expected:
Piston pushing redstone block should expand/retract whilst toggling signal.

Actual:
Pistons facing upwards pushing redstone block expand but do not retract. Pistons oriented any other way behave correctly.

Occurs with:
-13w02a
-13w01a/b
-Under both environments above

Notes:
-Occurs only with Redstone Blocks and only in the upward orientation
-See attached video