When the quest rule "Freeform dungeons" is unchecked, and lock blocks are used in place of an actual locked door, Link will stay stuck about 1-2 pixels away from the door, and thus cannot unlock it.
Some kinda solidity bug [635]
That is because without freeform dungeons, there is an invisible barrier around the room. Placing a door with the door tool is the only way to get through the barrier. So in short, not a bug.