Version: minihud-fabric-1.20.1-0.27.0

That's not a bug but intended behavior with the default settings.

Try enabling the config at Generic -> lightLevelCollisionCheck and then it should not show up there anymore. By default I don't do the collision check, because that will make the light level overlay update a bit slower/more expensive. (Although I haven't profiled how much that would actually be...)

Then overlay won't render under open trapdoor and door.