Related to and basically the opposite of MC-224494.

In seed 8321433975529524506 at 80 30 -40 you'll find grass blocks that generate naturally in an underground pool. This should not occur.

It was not caused by enderman damage, and the world was generated in 21w19a.

Similarly to MC-224494, the chunk loading order may play a role in whether you see this occur or not. When this occurred for me, I did not teleport myself there. I had my view distance at 10.