The bug

When a falling block (e.g. as sand) falls onto a non-full block (e.g. a slab or soul sand) without a block below it the falling block will keep floating and does not break.
However, if there is a non-replacable block (must not have a collision box) below the non-full block, the falling block will break as expected.

How to reproduce

1. Place a floating soul sand block
2. Let a sand block fall on top of it
   The sand keeps floating instead of breaking