It's almost impossible to forget to turn the cabin lights off, someone would have noticed on the odd occasion it might possibly have been. Most likely it is a mechanical issue, purely speculating but especially on the 747 this can be an issue as they're getting on a bit. If it's not that they don't go off, it can sometimes be a disco lighting effect in the F cabin. No harm in following it up with BA. I'd definitely want to know about it though as the CSM on the day so I can put it in the tech log for the engineers to look at, although I'd have clocked it on my walk around of the aircraft and if anyone was awake still would personally have invited them into the galley for a chat and see if there was anything I could do for them. Am sure most of my colleagues on here would do the same.