The bug
When falling blocks (like sand or gravel) fall onto snow layers which are stacked higher than one block, the falling block drops as item instead of replacing the snow layer(s), see also MC-143794.
How to reproduce
Stack snow layers higher than one block
Let a falling block fall onto it
→ ❌ The falling block drops as item instead of replacing (all?) snow layers
Can confirm in 20w51a.