What I expected to happen was...:
Grass blocks should replace dirt blocks if they are touching grass blocks and provided enough light.

What actually happened was...:
The blocks remain as dirt indefinitely. This happens throughout the entire world, spanning biomes. It does not affect newly generated worlds - just this world from 2017. The world's already been optimized and played frequently in 14.4.

FYI...:

https://bugs.mojang.com/browse/MCPE-41198

This seems similar. It is marked resolved in an older version, but the problem still exists in Java Edition 14.4. They also say coarse dirt does convert to grass, so it could be different.

Steps to Reproduce:
1. Find a wide stretch of grass blocks during the day

2. Shovel or remove a 2x2 section of grass, ensuring grass surrounds the new hole

3. Replace the empty hole with the dirt that's been collected

4. Surround the new dirt with torches or other bright light supplies

5. Wait an indefinite amount of time without pausing or moving location

6. The new dirt blocks never change to grass blocks