The bug
A fence connecting to an adjacent block placed by a player or moved in by a piston causes the observer to trigger, a tree growing besides a fence does not, even though it connects.
To reproduce
Place a sapling
Place a fence next to it
Place an observer facing the fence
Place a redstone lamp or a piston behind the observer
Bonemeal the sapling
→ ❌ When the tree grows, the observer will not trigger the lamp / the piston
Video
[media]Linked issues
Attachments
Comments

Did some more testing with other blocks.
The following show the same beahviour:
Walls
Iron Bars
Glass Panes
Bells
Leaves