On our multiplayer world we have build a sidewalk which has integrated glowstone lamps wired to a daylight sensor. Due to the length and use of redstone repeaters there is some delay to their activation when it snows and the sensor is triggered. During night the lamps get activated to prevent mob spawns, but during a daytime snowstorm a layer of snow is able to momentarily form on top of the lamps which prevents them from emmiting light and consequently making it unable to melt the layer on top or any surrounding it when it is activated.

Steps to replicate
1. In a snowy biome, place down a glowstone lamp, into the ground toggled off, so that only its upper face is able to provide light when in an active state.
2. Wait / Make it snow, until a layer of snow forms directly on top of the glowstone lamp.
3. Activate the the glowstone lamp discretely from underneath.
As a result the lamp goes into an active state, seemingly visually emiting glow but it is unable to penetrate the snow layer, being blocked by it as if it was a solid block.

I am unsure if this is intended behavior. glowstone lamps can melt snow while covered by a trapdoor or a carpet.