I was surprised that my partner, a BA Gold Member, travelling alone on BA SIN-SYD had selected a seat in economy where the adjacent seat was empty during OLCI, but was moved by Bag Drop staff an hour later without being informed, to a row that was full of passengers...only to discover this at the lounge where staff couldn't change it back, and furthur discover once onboard that her originally selected seat and the one next to it remained empty!
Mind you, hopefully she's learnt her lesson and will do as I did next time, i.e. paid extra to fly SQ (despite also being BA Gold).