Observers do not detect brewing stands finishing brewing

Steps to reproduce:

1. Place down an observer facing a brewing stand
2. Make sure the observer is connect to an unpowered block (e.g. redstone lamp)
3. Brew some potions using that brewing stand

Observed result:
The observer does not power the redstone lamp when the brewing stand finishes brewing the potions.

Expected result:
The observer powers the redstone lamp when the brewing stand finishes brewing the potions.