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 @unknown
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).
Please avoid duplicate comments. Post only NEW information regarding this bug.
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Heh, I wish I had known this might happen.
Just lost a fairly large snow structure to this bug.
Playing 0.13.1 Windows 10 Edition (PC)

Confirmed for 0.13.1 on iPhone 5s iOS 9.2

Still in 0.14 beta. Really worried this won't be fixed by the update. For me, it happens when its snowing.

Yeah, but this issue is from an iOS device, let's wait if this gets fixed

allready reported ( MCPE-12119) it happening on mine and my dads tablets and where using samsung galaxy tab 4 ...thats not an ios...

Even if its from an iOS device, if its not fixed in the beta its probably not going to be in the main update. Most bug fixes happen in the update and I have the same issue with an android device. If they fix it now, itll be working when they port the game to other stores.

also on the android devices 😞

hope the devs fix this soon!

So this bug can destroy a snow structure of any size, it makes an entire biome less beautiful, and it is still in the game after 6 months...
My theory (not supported by any source code) :
Since snow layers can stack when snowing in MCPE, huge stacks of snow can appear after a while. To avoid that, the devs decided that if a snow layer is on top of full snow layer block, that block should disappear at the end of the snow fall (as if the snow collapsed). Unfortunately, they checked for full normal snow blocks too.
Still an issue as of 0.15.3. Tested on Android Samsung Tablet.

0.15.7, still an issue, I hope this is fixed soon, I built a huge snow igloo and it disappeared (twice)

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).

Does this bug affect 0.16.1 or 0.17.0.1? If so please respond.

it appears to be fixed when chunks loading/ unloading but not when the rain stops
tested it by building snowblocks making it rain then jumping in a bed and the snowblocks was gone
This ticket has been resolved as 'Cannot Reproduce' as it has no response was received recently.

Apparently it still occurs as of 1.18.12, I just had all the fallen snow around my base disappear after I came back from the nether. Although the player placed snow/snow layers stayed

1.20.30Â Switch OLED, Game Card
Thumbnail of the world still shows full snow layers.
Now it's a random mess of varied layers (as if it snowed for the very 1st time in these chunks) when I log in to check my backup worlds.

Issue with snow blocks solved, still affects snow layers.People who to experience with snow layers this can go to the report MCPE-150476.