I could be wrong but I think your more than likely the victim of what they call in the house buying market “gazumping” and the dealer had a private buyer willing to put money up front for your car that far exceeds anything the dealer would make on a Motability sell.
As a Motability customer there’s nothing you can do about it.
As for ending up at the back of the queue again I think there’s no doubts your already at the back of the queue again.
Where folk here talk about walking away from an order anytime up until you input your PIN when collecting your car it works both ways but I would like to see a tightening of the rules that are a little bit more binding on both sides when ordering a new car though I’m sure a lot on this forum would disagree with that.