I’m guessing the room on the other side of this door is at that lower level. If so, it would have been a significantly better idea to build a small step on the other side of this door and hang the door so that he bottom lines up with the rest of the floor in this room. That way they would not have had to cut out part of the floor in this room. This seems like a much worse tripping hazard than having to step up into the room at the door frame.