Both the door and trapdoor are blocks which are three pixels thick. It is possible to place a door on top of a closed top-half trapdoor. When said trapdoor is opened, the door will pop off, which makes sense in most cases, as the door block simply cannot exist without support. However, the door will still pop off even if said trapdoor is aligned to the same block face as the door, which would mean it has just enough space to support it. One would expect the door to not pop off in this case, and also be placeable.
For similar reasons, it is not possible to place doors on top of other doors.
Linked issues
is duplicated by 1
relates to 9
MC-146824
Inconsistency: Ladders and tripwire hooks cannot be placed on the sides of redstone blocks, observers and target blocks
Resolved
MC-131115
Torches, levers, buttons, tripwire hooks, coral fans, vines, ladders and bells cannot be placed on the sides of farmland or grass paths
Resolved
MC-146928
Can't place doors, rails, buttons, pressure plate, redstone, etc. on soul sand
Resolved
MC-147248
Wall torches cannot be placed on the sides of lower slabs or stairs
Open
MC-147249
Bells cannot be placed on the sides of upper slabs, stairs, the side of a piston base, or the stone side of floor bells
Open
4 more links
Was able to re-create this on Minecraft 19w14b.