I think something happend to break a few trackers that track across characters. You will also find some threads on reputation trackers being very broken. As in, you have the 80 rep achievement but when you look at the 90 counter it only shows 25 of 90 reps. (I should be one rep away from 100 but still showing only i think 27 reps of 90 in my counter)
They did finally say something about reputation trackers being broken.
I hope they are looking into your mount collecting too.
My usable mount count for the achievement has been stuck at 289 even after buying a mount and earning two more from a raid glory achievement. I logged into every toon and nothing updated. After reading that adding a new mount on the character with the most usable mounts can trigger the count update, I decided to try building a trial paladin since it would theoretically have access to all the same mounts as my other paladins. After building the trial pally, the count didn’t change immediately, but when he finished his starting quests, he was awarded the troll pally mount and that triggered the update and now I’m at 291 which is at least a couple steps in the right direction.