The bug

By putting a redstone block then a piston then a grass block in a diagonal line going downwards you can place a snow layer and update the piston to create snowy grass.

https://youtu.be/C1k76xOB3K4?si=FuVVanms4t62nDI9

How to reproduce

1. Place blocks as shown in Setup.png
2. Place a snow layer on top of the grass block
   → The snow layer is pushed by the piston
   → The grass block still appears to be snowy