Grass blocks underneath mob spawners turn into dirt

Summary:
Grass blocks underneath mob spawners turn into dirt.

Steps to Reproduce:

1. Place a mob spawner on a grass block
2. Set the random tick speed to a high number

Observed Results:
The grass block turns into dirt.

Expected Results:
The grass block should not turn into dirt.

Video:
Minecraft 2020-12-31 12-14-31.mp4