The bug

Gravity-affected blocks only break and drop as items when landing on powder snow from specific heights. This is an unique behavior, and I'm not sure whether this is intended. According to MC-218738, it is intentional that falling blocks don't fall through powder snow. However, they convert into solid blocks when falling from under a certain amount of blocks, and break otherwise. This amount can be anywhere between 3 and 6 blocks.

Expected behavior

Powder snow should either always convert the falling blocks to solid blocks (as solid blocks do) or always break them (as non-full blocks do), no matter the distance the falling block falls.

To reproduce

1. Place a slab.
2. While on top of the slab, run

/summon minecraft:falling_block ~ ~0.51 ~ {Time:100}

3. Notice how the block breaks instantly.
4. Place powder snow and stand on top of it with leather boots.
5. Run the same command.
6. Notice how the falling block converts to a solid block.
7. Run

/summon minecraft:falling_block ~ ~3 ~ {Time:100}

8. Notice how the falling block breaks.