The bug

When a snow layer of any size falls onto an eight-high (full) snow layer block, one snow layer is lost in the final stack. Dropping one layer onto the block causes it to disappear entirely. Dropping a stack of five layers onto the block causes a stack of four to form on top, and so on.

The proper amount of layers are added when they fall onto any block I tested other than an eight-high snow block. For example, dropping three layers onto a seven-high snow block correctly finishes the snow block (making it eight-high), then adds two more on top.

How to reproduce

1. Place eight snow layers to form a full block.
2. Above it, place any solid block.
3. On top of the solid block, place one or more snow layers.
4. Break the solid block.
5. Observe the height of the snow layers on top of the lower snow block. It will be one less than expected. If you only dropped one layer, it is lost entirely.