When trapdoors are placed at the top of a block, mobs can spawn on the following types:
Warped trapdoor
Crimson trapdoor
Iron trapdoor
Mobs do not spawn on top the following top-placed trapdoors:
Oak trapdoors
Spruce trapdoors
Jungle trapdoors
Birch trapdoors
Acacia trapdoors
Dark oak trapdoors
Mobs did spawn on top of top-placed trapdoors in 1.14, 1.14.1 and 1.14.2, but it was changed in 1.14.3 I believe it was. Not sure if iron trap doors have been spawnable ever since, or if that was re-introduced in 1.16.
Personally, I think mobs should be able to spawn on top of them, when they are placed in the top of a block and are closed.
It's kind of the same as other non-full blocks placed in the top position, like upside-down stairs or a half slabs in the top position.
Either way, it should be consistent with all trapdoors.
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-181233, which is a report that has not been resolved yet.
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