So I took an observer and hooked it up to a melon stem on hydrated soil. I used bonemeal, an as expected, the observer did detect growth. Then I took a melon and put it next to the stem on some dirt, expecting the observer to trigger. It however didn't, which I think is weird (there is a block state change going on, which an observer should detect).

Steps to Reproduce:
1. Place a block of soil, a block of dirt next to it. Make sure the farmland is hydrated.
2. Plant a melon/pumpkin on the farmland, and place an observer onto it. Make sure you can notice an observer pulse with a piston or lamp.
3. Bonemeal the stem. This should pulse the observer.
4. Place the corresponding block (melon/pumpkin) on the empty dirt, which connects to the stem. The observer however doesn't notice the "facing" block state change.