Grass generates on blocks they can't be put on.

I was playing around on the 1.20 snapshot when I found this seed (below) that had a ruined portal and I noticed that the grass was on the netherrack. I middle-clicked a piece of grass and tried to place it, and it didn't place on the netherrack.

 

Notes: I'm 99.99% sure it happens on 1.19.4 and under too.

 

Example Seed: 579358382962016144

Seed on Version: 23w12a

Seed Picture