Rocks (also twigs and probably others too) that generate in snowy biomes replace snow but leave the grass block below as snowy.

(this is what a snowy grass block without snow on top looks like)

Found these also:

Seagrass and lily pads generating in frozen rivers. Caused by enableExtraLilypads and enableExtraSeagrass:

I think I might've fixed these, or at least most of them I hope