Iron Trapdoors pushed by pistons keep their state until they get a redstone update.

Iron trapdoors pushed by pistons while being triggered keep their state until they get updated by powered blocks(and unpowered again) or unpowered redstone. Block updates do not change their state. This bug is caused by the fact that iron trapdoor's code it's pretty much a carbon copy of the wooden trapdoor's. This may work as intended , giving players a new type of wall cover. This bug it's harmless and doesn't break any features or affect the gameplay.