Dirt is caused under the hopper and mob spawner blocks, hopper is not covering the grass fully, and mob spawner allows light for grass.
How to re-produce:
1: Go into a flat world, place down hopper and mob spawner.
2: Set ticking speed to 9999, then exit out and go in again, then set it back to 1.
3: Notice how there is dirt under the two.
Thank you for your report!
We're tracking this issue in MCPE-113305 and MCPE-62132, so this ticket is being resolved and linked as a duplicate.
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