In 1.16.1 it worked for sure (at least on iron trapdoors, I've created passive spawn farm using them). And don't remember any troubles in 1.16.2
Maybe I used wrong words in description, I hope attached screenshot gonna help
Trapdoors and slabs are on the same level here, but spawning works only on slabs. Previously it worked on trapdoors too
Ostap Plaksii
In 1.16.1 it worked for sure (at least on iron trapdoors, I've created passive spawn farm using them). And don't remember any troubles in 1.16.2