I just reported this in-game and I didn’t copy what I wrote so I’ll try to remember. Basically, I main this druid and I have a paladin alt. I’ve determined that my pally is the character who has the highest amount, so whenever I would learn a new mount on my druid, I would log over to the pally and the in-game tracker would increase. Ever since 8.2 or 8.2.5, it stopped working. I would log over and the number wouldn’t increase. I was stuck at 338/350 for at least a week or two, through weekly reset.
I also checked the Wowhead tracker and the Armory for both characters. My Wowhead tracker showed like 335 on my pally, which I thought was weird, but I took it with a grain of salt because technically third party, even though they pull from the Armory data. On the Armory, I checked the specific mount achievement and saw that it said 341. I was bummed that it didn’t show 341 in-game, but I was hopeful that it would sort out.
Today, I was on my pally and learned a mount on my pally. The in-game tracking number went from 338 to 342 immediately. After that happened is when I submitted my bug report.
Hopefully those of you who have also experienced this can try learning a mount on your specific character (the one with the highest amount of mounts) and see if that helps update the in-game tracker.