Pistons don't push if given a signal before obstruction is removed

The block switcher on the survival server I use stopped working after updating to 1.5. I checked my identical version on singleplayer creative, with no success.

What I expected to happen was...:
If an "on" signal is connected to a piston that is obstructed (ie obsidian) and said obstruction is removed, it should automatically activate.

What actually happened was...:
As long as there is a block between the piston and the obstruction, the piston will not active if the obstruction is removed (provided that an "on" signal is supplied prior to removal).

Steps to Reproduce:
1. Place a piston facing any direction
2. Place a moveable (ie dirt) block on the wood side of the piston
3. Place an immobile block (ie obsidian) next to that in the same direction
4. Give the piston a redstone signal
5. Remove the immobile block