Hi Wkoziol,
The first time I built the room I had started to decorate it and then added additional walls etc. The second time I tried to replicate the issue for testing purposes, I didn't add any decorations until later.
It wasn't until I added quite a few different walls etc that the problem arose again and I could not work out how I managed to fix the issue in the first place.
The room/s were all on the ground floor.
I tried everything I could think of to get the room recognised as one room: deleted all walls and rebuilt them separately (lead to them all being recognised as individual walls and not a room at all), not decorating it, decorating it etc etc. Nothing worked for me. Luckily I had already managed to blindly solve the problem in the first instance so the second room was just out of curiosity (and to hopefully help any one else if they got the issue).