Placing snow layers against the side of snow layers only works on full snow layers and will (re)set the new snow layer block's height to 1

How to reproduce: Stack a snow layer block to full height, place a snow layer against the side of that. Try placing another one, it will fail. Place a single snow layer on the new snow layer block (the lower one) and attempt to place a snow layer against the full height block again, the two high one will now have reverted to being one high and consumed a snow layer block from your inventory if you're in survival mode.

It is also impossible to place snow layers against the side of existing non-full snow layers, attempting to do so will only increase the existing snow layer block's height until it is full, at which point the behavior described above will take place.