On a flight last month while I was boarding I heard a lady ask the GA if all the upgrades had been cleared and the GA said yes. After most of the plane had boarded one F seat in front of me had remained empty at which point I saw a FA bring that same woman from Y and sit in the seat. Just moments later though the rightful owner boarded the plane and the lady had to get back to Y.

Perhaps the FA saw it was past the 15 minutes before departure and asked the GA and was given the seat of the passenger next in line? I've also been in the same situation as that lady and it really sucks, especially when you're flying an int'l airline with a much nicer business cabin!