The bug
Having a observer facing away from a grass block does not detect when a tree grows, changing the grass to dirt. It does detect if a block placed causes the grass to change and it does detect if the grass grows back.
Related issues
Comments

Duplicate of MC-107664
Can confirm in 21w08b.
Can confirm in 1.16.5.

can confirm in 21w10a
Please fix this. You fixed the issue with the walls, trees, and observers. This seems like something that would be along the same line. This is a major issue that is keeping me from being able to finish my tree farm. 😞