Some might say that this is irrelavent. But bear with me: it's still a bug. Other doors work on DMAP 0. Boss doors don't.
No Boss door on DMAP 0
Oh, I bet I know why! 😊
I bet DN organized the Boss Keys in a big ol' array with as many cells as there are levels. And that he forgot to start the indicies of the array at 0.
Am I close?
-C