Snow melts with no daylight and no block light

I was trying to create a zoo, with Top Snow blocks (Snow Layers), but they're melting even though there's a ceiling (no daylight) and no light sources (there are monsters spawning). This zoo is being built on plains biome, so the climate should be temperate.

Steps to reproduce:
1 - Go to a plains biome
2 - Build a 10x10 square with ceiling
3 - Fill the inside with 1 or 2 layers of Top Snow blocks
4 - Room should be dark and filled with snow layers
5 - Wait and watch the snow melt

Expected result:
Snow should stay there, so it can be used as decoration, which is seen in some villages

Obtained result:
Snow melts with no daylight and no block light