Retracting Sticky Piston Arm doesn't trigger Observer Block

Steps to reproduce:

1. Place a sticky piston in a position where it can extend.
2. Place a block in front of the sticky piston.
3. Place an Observer block facing (watching) the block placed on Stem 2.
4. Remove the block placed on Step 2.
5. Place a Redstone Lamp (or any other redstone activated block) on the output face of the Observer placed on Step 3.
6. Power the sticky piston placed on Step 1 with a lever (or by other method.)
7. Unpower the sticky piston.

 

Expected Behavior:

The Observer block triggers with both the extension and the retraction of the sticky piston.

Replacing the sticky piston with a non-sticky piston produces the expected behavior.

 

Current Behavior:

The Observer block is triggered as expected with the extension of the sticky piston, but not with the retraction. This is inconsistent with behavior of non-sticky pistons, which do trigger Observer blocks when retracting.

Additional Notes:

Quasiconnectivity BUD's and blocks in invalid positions (such as floating Sand/Gravel) are updated properly with the retraction of sticky pistons.

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