Same thing for me on 3 upcoming flights over the next 6 months, so it does look like a new policy. Either that or maybe BA updated whatever software runs MMB and/or the seating interface and forgot to turn off the cot-seat block for the upper deck when they rebooted the system.
In two cases I have been moved to 64K and in the other case to 14A (presumably as 64K is already taken, but maybe the re-seating algorithm thinks I need to shake things up a little.