According to the Wiki mobs don't spawn on blocks with 2-7 snow layers on them, but I'm seeing that happen. Went into a creative world to test it and they're spawning on blocks with 2-4 layers (didn't bother testing 5, 6, and 7) on them. I even set up a box with only 2 blocks of height to spawn in (not counting the snow) and covered all of the blocks in 4 layers of snow, and the mobs still spawn and immediately suffocate until they die (could be an easy mob farm but I assume this is a bug). I found this out after covering my mountain base in 2 snow layers to spawn-proof it without lighting, seems like this can really mess up some builds that use this mechanic for spawn-proofing.