When Placing a block above a trapdoor (doesn't matter if iron or wood), the trapdoor falls to the ground like if you mined it.
When Placing a block above a trapdoor (doesn't matter if iron or wood), the trapdoor falls to the ground like if you mined it.
This issue has been reported already. It has been linked to the original bugreport.
Please search before reporting any bugs, as it's likely that one exists already.