If a snow block in a snowy biome is exposed to the sky, it and any snow blocks beneath it will eventually disappear, the uppermost being replaced by a stack of snow layers. It has been suggested that this occurs only during or after snowfall. This information fits with most current observations, however, they may also be replaced following the unloading/reloading of their chunk.
For ice plains spikes biomes, which generate snow blocks in the place of grass, this happens immediately upon generation (or upon reloading a world last opened prior to 0.12.1).

Shown is a player-built igloo in an ice plains biome (old-type world), before and after the snow blocks were replaced, in addition to a newly generated ice plains spikes biome (infinite world).
All snow blocks in a chunk appear to be ticked at the same moment, assuming the required conditions are met. It also seems as though nearby snow layers are regenerated during any occurence.

Update by nighter
This bug has been reported to our internal bug tracker for further testing and a fix. It is scheduled to be fixed in one of the future updates (no specific date can be provided).
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