Motability are playing the waiting game in hope, seeing your into your forth year, that you renew the car before the expense of four new tyres.
I had this with my front brake pads when on its first MOT there was an advisory of low pads and Motability declined the renewal because I had a new car on order. As it turned out the new car took 15 months to be delivered and that meant a second MOT a year later where the service department gave it an instant fail and just to add a twist to the knife in my back, notice my back and not MB, they kept the car in over night because they said they never had the pads in stock so I was left high and dry with no car all because MB said no the year before.
It turned out that during the fourth year while waiting for the new car I had to have two new tyres as well so these car delays cost MB as well as our patients.