Redstone Blocks above Pistons fail to retract vertically

I observed the following in single player and multiplayer: Pistons, sticky and regular, fail to retract back down when a redstone block is above the extended face. Therefore, redstone blocks can be pushed down, but not pulled down.

What I expected to happen was...:
1.) Upward-facing pistons are powered.
2.) Pistons extend up. A redstone block is above the face of the piston arm.
3.) Power is removed from pistons.
4.) Pistons retract. Sticky pistons pull the redstone blocks down.

What actually happened was...:
1.) Upward-facing pistons are powered.
2.) Pistons extend up. A redstone block is above the extended face.
3.) Power is removed from pistons.
4.) Pistons fail to retract, as they do in every other direction (horizontal or down).
Note: Pistons only retract if redstone block is removed.

Steps to Reproduce and Observe Inconsistent Behavior:
1.) Place an opaque block with three blocks empty on the top, bottom, left, and right sides.
2.) Place a piston or sticky piston on these four sides, facing away from the center block.
3.) Place redstone blocks on the faces of each of these pistons.
4.) Power ON the center block and allow the pistons to push the redstone blocks.
5.) Power OFF the center block and observe that all but the upward-facing piston retract.