When the iron trapdoor is drooped from the inventory, like with Q (default), it looks like stone, and then fixes itself back to an iron trapdoor.
MC-48830
Iron Trapdoors render bug as item
Resolved
Duplicate
0
This is up to your PC.
Every item that you drop will first be rendered as stone, but then it realises it isn't stone so it changes to the item it has to be.
The time it takes to change to the good item depends on your video card speed.