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.
Environment
Windows 10 build 2004.
java version "1.8.0_251".
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.
If you want, you can upvote the parent ticket, and enable watching to receive any updates about it - if you believe you have any information that may be important for this issue, please leave a comment on the parent. Please also use the search function in the future to prevent duplicate reports.
Quick Links:
📓 Issue Guidelines – 🛠 Community Support – 📧 Customer Support – ✍️ Feedback and Suggestions – 📖 Game Wiki
💬 Mojira Subreddit – 💬 Mojira Discord