The Bug
When two blocks of grass are touching the connecting textures connect with each other and begin to z-fight
https://youtu.be/5d7Rk3DDZjM
How to reproduce
The seed of the world where I encountered this issue is 7534800333963896490 and the co-ordinates are 55 73 137
Related issues
Comments

It's similar and it's possible that's the same root cause. I didn't go to that report however since that was describing tall grass while I experienced this with the normal 'grass' block.
EDIT: Seems like it's the same cause so I've resolved this as a dupe. Thanks Marty 😃
Does MC-130709 describe your issue?