This all has to do with the side texture for grass/snowy grass/dirt etc.

When snow(block and pile) is placed or spawns naturally on grass it turns the grass into snowy grass. This looks great and helps transition the white to the brown nicely. However the snowy grass eventually just turns into dirt, which ruins the look.

I noticed that a mountain near me didn't look all that great because of the stark contrast of the snow and dirt and I remembered there being snowy grass between them at one point but I might be misremembering. Pics below show the issue.

Fixes-remove the decay timer when snow is placed on grass making it always display snowy grass.

Optional Idea: also make regular dirt use the snowy grass(side) texture(when snow is above it) because it makes a nice transition from snow to dirt. (otherwise its working as intended.)

*EDIT* Confirmed to be vanilla behavior. However it may not be intended for the visuals of some biomes to be (subjectively) off looking.