Observer can't detect the change of facing of Pumpkin/Melon Stem

Put the summary of the bug you're having here

What I expected to happen was...:
Observer gives redstone output when the Stem changes visually by merging into the last Melon/Pumpkin spawned

What actually happened was...:
Nothing

Steps to Reproduce:
1. Plant a Melon/Pumpkin Stem
2. Place an Observer facing into it
3. Wait until the Stem spawns a Melon/Pumpkin or just place one near the Stem