The Bug
I was traversing the nether in a world I had previously played on 1.17. After moving it to 21w41a, I noticed normal stone blocks in the nether, and these were new chunks that had just generated.
Reproduce
Go to the Nether
Travel around in the Nether looking at gravel patches
❌ Notice in some of the gravel patches became stone
Expected Result
Stone wouldn't generate in the nether from updating gravel
I was able to reproduce this is because there is a chance that naturally generated gravel that can update will convert to stone to prevent lag.