Motability state “If you need to pay an AP, the amount is fixed when you order your car.”
https://www.motability.co.uk/how-it-works/payments/
Its at the sole discretion of the supplying dealer whether they will cancel and reorder the vehicle at the lower AP!
Most dealers will, as its only admin on Motability’s side. However if a dealer refuses, you are perfectly entitled to cancel and reorder the vehicle from the same or other dealer.
Its always worth asking at the point of ordering, if the supplying dealer will honour this, as it saves any hastle later, should the AP go down.