When walking from a top trapdoor onto a scaffolding block the player just falls right through the scaffolding as if it wasn’t there. This happens with all trapdoors, including iron.
Walking from trapdoor to scaffolding behaves like every other exactly full height block. Allowing the player to freely walk onto the scaffolding.
1. Place a top trapdoor.
2. Place scaffolding next to the trapdoor.
3. Walk off the trapdoor onto the scaffolding.
4. Observe that you phase right through the scaffolding.