I’ve never had the AP go down while waiting for a car but did have the car taken off the Motability list a year before the car was delivered.
From threads and posts here about members have had their AP’s lowered in respect of it going down else wise they stick with the AP on the order day if the AP goes up. If the AP is lowered while waiting for delivery you will need to contact the supplying dealer and ask them to reorder the cars as its not a forgone conclusion they will automatically lower it, In fact they won’t.