Snow Layers (Snow), do not let light pass through them, meaning if there is a snow layer on top of a light source, and that light source doesn't have a way "around" the snow layer on top of it, it will never melt.

Replication and demonstration is very simple:

1. Create a superflat world with the preset:
minecraft:glowstone,minecraft:snow;minecraft:plains

2. Break one piece of snow:

3. Observe as the snow melting spreads: