This bug has been presented in the Java Edition where the observer does not get updated when the fence has connected to a full grown tree. This same bug is now presented in the Bedrock Edition.
How To Reproduce:-
Step 1:- Place a Observer Block.
Step 2:- Place a Redstone Lamp in the red dot of the Observer.
Step 3:- Place a fence in the Observers Face.
Step 4:- Place any block to connect the fence/bone meal a sapling.
The Observer does not get updated when the fence is connected to another block or when sapling is full grown into a tree.
The Observer should detect the update.
Hoping this information is helpful to fix the bug.