This happened to me for the return leg of my LHR-PHX flight a few weeks ago, the seat map was originally there and I was able to pick 1K and 2K for my wife and I, then a couple of days before the flight the seat map (or lack of it) was identical to what you are now seeing.

The other anomaly I had was that 1K and 2K are both window seats yet on the booking it had 1K (middle) / 2K (middle).

Called BA and the agent said that they could see the seat map fine at their end and could make changes for me if I wanted, they confirmed that my seats were window and not middle.