As DumfriesDik says, Motability need to change their rules to offer the ordered AP or pick-up AP, whichever is lower. I would definitely be tempted to wait but they may just deliberately stick to the rules and refuse to adjust the AP if it goes down. That may then leave you with the option of paying the current AP or make you completely re-order and give your car to someone else. This would be a very extreme thing for them to do but it depends what your dealers like and whether it’s an in demand car they could shift immediately.
You should give Motability a call and ask them about it. They must know Q3 prices by now.