This is associated with issue #102.
"Spawn Medium": "air" may be preventing spawns on snow covered blocks. I have observations of a world that stopped spawning on snow covered blocks in recent updates. Upon changing the spawn conditions to:
"Spawn Tag": "!solidside(1,{0,0,0},{0,-2,0})||liquid({0,0,0},{0,0,0})||normal({0,0,0},{0,-1,0})||normal({0,0,0},{0,0,0})||normal({0,0,0},{0,1,0})||!opaque({0,0,0},{0,-2,0})||!air()"
This resulted in spawns that were spawned one block in the air above the snow covered block. I'm assuming that the block with the snow layer in it isn't considered "air" as a spawn medium.

I just wanted to confirm that there are spawns on snow covered blocks that are otherwise permissible for spawns. I tested with v0.16.18. I haven't exhaustively tested that there are spawns on blocks that have decorations, such as flowers and modded foliage but so far I have seen little reason to believe that this is an issue. Thank you for your quick attention to these matters!