If you leave it a day it may well gravitate to whatever is in your profile preferences. You appear to be the victim of curtain roulette, particularly if you are within 72 hours of departure. If so, then the algorithm in Altéa Seating may not have run and if so, Theoretical Seating is dynamic until you positively select a seat. If you are more than 72 hours from departure then either the algorithm has failed, you haven't given it enough time, or a very large number of people were moved and at the time the options were limited. They use the same email for both Altéa Seating and Theoretical Seating (and that is reasonably new too).

Obviously curtain roulette can still be played right up to check-in at MXP/BGY/LIN.