How to replicate: create a floating floor of solid blocks and replace a block in the center with a trapdoor. Walk (not running, just walking) over the opened trapdoor.

Expected result: falling down, since the trapdoor is not closed.

What happens instead: you walk over the trapdoor as if it was closed.

Even mobs don't fall.

Unless you stand still over the trapdoor, you won’t fall in the gap.