I forced the weather rainy and I went into my hidden place and the rain hit the open trapdoors and "I didn't get wet" and the rain hit the open trapdoors as if it was closed.
Linked issues
is duplicated by 4
MC-105455
Rain Splash Particles sill appear on top of Trapdoors when they're closed
Resolved
MC-237616
Zombies gets burned out during rains when non solid block covers the water
Resolved
MC-254504
Rain does not fill toilets with water unless you remove the toilet lid
Resolved
MC-270376
Player keeps burning when hit by a sword enchanted with Fire Aspect next to an open trapdoor on rainy day
Resolved
This is because every (i think almost every) AI in minecraft sees an trapdoor always as a solid block, since solid blocks can't let rain trough, trapdoors also won't.