Happened to me on the return leg a couple of weeks ago, picked new seats when the curtain was moved, noticed later in the day in the app it still showed the original seats and then looking at the seat map, the new seats I'd chosen were greyed out. Confused I selected seats on the opposite side and on saving it still showed the original seats but now the new seats I'd picked earlier were showing as available, so moved back to those again.

A dummy booking showed the original seats as available so it looked like the new seats had been assigned just the excellent AFKL IT was lagging somewhere. Later the next day the booking finally correctly showed the new seats...