Trapdoors behave as though they have a hitbox one pixel smaller than a regular block, on their flat side.

If you place a "wall" of trapdoors on top of a regular sized block, you will still be able to stand on the edge of that block, despite it visually appearing aligned as though they should have the same edge. This allows you to stand on the edge of a block despite looking like a flush wall, and also can cause mobs to get stuck in chutes made of trap doors, if there are standard blocks anywhere along the side.

Mobs or players landing on this invisible edge will take fall damage as normal.

This extends to blocks above the trapdoor as well - if you place two trapdoors as a "wall", then another block on top of them, and stand on the edge of the trapdoor, then jump, you will hit your head on the top block. stacking additional trapdoors, or nothing above the trapdoors, allows normal range of jumping.