Falling blocks that are removed before they can remove the block that spawned them create ghost air blocks

Falling_block entities that are removed from their location before they can remove the blocks that spawned them lead to the original gravity affected blocks becoming ghost air blocks. That is to say they become invisible to any present clients, but are still there as far as the server is concerned and show up again upon a reload. This can be easily reproduced by killing or teleporting the falling block 1 tick after it was created.

In the depicted example, the command in the command block is "kill @e[type=minecraft:falling_block]"