When you want to protect yourself by lava, you replace fast lava sources by blocks. But when you dig and replace fast ancient debris, you lose these.

How to Reproduce

1. Dig a 1x2 hole 2 blocks beneath the surface ground
2. Mine a block above you, then quickly replace it with another (best way: offhand)
3. You will not receive the block you mined - it will instead clip to the nearest surface above (see screenshot, blocks clipped through the nether blocks)

Expected: The block drops downward to the player, so they can collect the item.
Observed: The item goes upwards, and lands at the surface where the player cannot pick it up.