This is really minor, but I checked the ROM and this is inconsistent.
First, go to a room with a bombable wall and then work your way to the other side of the bombable wall without breaking it. Now, from the other side, bomb the wall and go to the subscreen. On that brown map that "carves" each room out, you will only see a one way connection to the room on the other side until you re-enter the room on the other side. In the NES version, you will see a two way connection. This is easily reproducible in Level-1 in 1st.qst. Go to the third room up from the entrance, work your way to the room with the map, and bomb the south wall. Pull up the subscreen and enjoy the minor bug.