In both 1.12.2 release as well as 17w50a, snow layers can be placed as well as forms naturally when it's snowing in according biomes on top of Command as well as Structure Blocks.
I'd have expected that blocks which are only obtainable and accessible via Creative means would not be affected by anything as usual (e.g. also explosions, fire).
Apologies if this was already reported, I searched around but could only find MC-117063, snow layers on barriers, which is not WaI according to Mr. Broes on MC-2813.
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[Mod]Les3awe
violine1101
I don't see why snow falling on solid and opaque blocks would be a bug, regardless of whether they're available in Creative mode only or not. I especially don't think that placing snow on command and structure blocks should be prevented.
This is a feature request and doesn't belong here on the tracker.
I get what you mean and it's up to you mods to decide whether or not Mojang would see this as "WaI" or not, but I personally disagree, as those are mapmaker tools. Certainly, carpets should still be placeable on command blocks, as some differentiate various chains/mechanisms like that; but snow layers are for me personally on the same level like any other form of "natural events" (or catastrophies) or technical components (e.g. pistons pushing) which neither mapmaker tools incl. e.g. Marker-true-Armor Stands are or should not be affected by, thus snow layers seem logical to me to be included here, but it's certainly not that much of an importance, so I'm not going to argue for much longer about that, I'll leave that up to you and accept that decision.
(Unrelated side note: There are labels "snow-layer", "command-block" and "structure-block" (and some more with a "-"), and from how I recall on another bugpost/explanation by someone of Mojira, those should be all written with an underscore, thus I only used those "underscore-labels" and did not use the others, so maybe they should be deleted if this is not the preferred label method here?)