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

1. Place a sapling
2. Place a fence next to it
3. Place an observer facing the fence
4. Place a redstone lamp or a piston behind the observer
5. Bonemeal the sapling
   → When the tree grows, the observer will not trigger the lamp / the piston

Video

Minecraft 20w13b - Singleplayer 2020-03-29 12-43-07.mp4