Using a Grass Block (minecraft:grass_block) on a single snow layer (minecraft:snow[layers=1]) causes the Grass Block to replace the snow, which is expected. The Bug is, that the Grass Block is then set to [snowy=true] without having snow on top of it.
Can confirm for 20w18a (macOS). It seems to be an actual bug (source) as the F3 screen displays it as
snowy: {color:#00875a}true{color}
and theclone
command creates a snowy grass as well. Reloading the textures, chunks and/or the world does not fix the issue. Updating the block by placing other blocks does not work, except for placing them directly on the block. The block can be fixed by moving it with a piston, however.May be indirectly related to MC-160797.