I am having this problem as well. I was 8 mounts away from 400… I got both mounts from the 15th anniversary and instead of only being 6 mounts away I am still 8 mounts away. My mounts count is also showing up correctly on wowhead, just not in game.
*Update: I also purchased a mount in game and this put my achievement number at the correct amount. But like it has already been stated, we should not have to do this.
I also encountered this. I earned the anniversary event mounts on another character, and when I logged into this character to update my achievement tracker, my mount count remained unchanged at 383/400 though outside websites and achievement tracker addons showed 385. I purchased the Xiwyllag mount and learned it on this character and then my count jumped to 386/400.
Edit: learned a mount from Mechagon and the counter still says 386 on this character instead of 387
This is still happening. The mount counter doesn’t increase when learning a mount on my druid and logging over to my pally, as it used to. I’ve taken to earning mounts on my pally, instead, which is a hassle but doable. For example, the other day I got a mount while not on my pally (which didn’t increase the counter, even when logging over), then earned enough Champion’s Seals at the Argent Tournament on my pally to buy another mount. When learning that Argent Tourney mount on my pally, the counter jumped up 2 numbers from 353 to 355.
I reported it over 6+ months ago… They have done nothing… Enjoy!
Make sure you log onto ALL of your toons that may have class-specific mounts, ie. Pali or Warlock.
For example, I’m at 466, but some of my toons had shown 465. I had to make sure I loaded my Paladin and Warlock to capture mounts that were exclusive to them. I had to do this when I was chasing the achievement myself. Just make a note to log onto each toon and remember which ones change your count number. It’s annoying, but, long time known problem.
no, this is a new problem.
i have the highest mount count on my paladins.
got 3 class mounts on a paladin, watched the achievement tick 297, 298, 299…
then went and learned:
Brinedeep Bottom Feeder and Ratstallion on another character.
logging back to the paladin, it didn’t change the mount count.
(also logged other paladins, warlock, engineers/tailors just as a precaution)
achievement count stayed at 299.
Did a 100 badge grind on the paladin to get Argent Tournament mount.
…the achievement for 300 mounts popped, and the mount count instantly went from 299 to 303.
the game needed the mount to be LEARNED on the highest mount-count character for them to register toward the total, and refresh the count.
I am having similar problems.
A few weeks ago, I earned the Stormpike Battle Charger/Frostwolf Snarler from Korrak’s Revenge, but my mount total in the “No Stable Big Enough” achievement progress remained at 301.
I logged on and out of every. Single. One. of my characters, and it still didn’t update. My count remained stubbornly at 301.
I went through a few ticket reopenings trying to get to the bottom of this, and was eventually told that they could not help me; my highest character should be Mahnta-Bleeding Hollow, a 54 Horde Paladin, but logging on to her didn’t work, and they couldn’t fix this for me manually, so I should submit a bug report and hope for the best.
I no idea why Mahnta is my character with the highest number; I assume my Horde Rogue LW with 2 prof mounts all 3 class mounts should have been, or even my Alliance Druid I’ve mained for 13 years. It happened quite suddenly several months ago that I had to log on to Mahnta to get the total to update, and I hadn’t used her since freaking Cataclysm nor have I earned any Pally class mounts! Yet another bug?
Fast forward to just now. Yesterday, I earned the Obsidian Worldbreaker. Did this perfect, cross-faction mount give me a new tally in my achievement progress? Absolutely not. Even after logging on to my Horde characters my number did not go up from 301 to 302, let alone the 303 I should have by now.
My only idea is that perhaps none of these counted because I earned them on an Alliance character. Who knows? Perhaps it is a problem with the Anniversary mounts. Perhaps my account/characters are broken in some way.
I’ve read that Blizz fixed a bug where people’s anniversary rewards were unusable. Perhaps they didn’t fix it all the way? 
In any case, at this point I’m scared to earn any more mounts for fear they’ll get lost in the shuffle somehow.
Wow, this had better not be the solution because I am NOT rerolling just to change a number.
Glad I checked back here before I bought squeakers today.I almost bought it on my main then remembered this issue.
Logged onto my paladin, after buying wow tokens on my main to get funds into my bnet account, to buy it to ensure the count went up.
Sad that this issue is still occurring. I am 17 away from 400 and it is sad that any mount I get on my main it not going to count. At least not until that number surpasses the number of mounts my pally has at the moment anyways.
Yeah, I got a Yeti to drop today on my Horde Rogue LW (second highest mount total) and the count went up by one to 302… NOT all the way up to the 304 I should have by now. so getting a new mount is not necessarily a fix. As we know it is not possible for me to farm mounts on my inexplicably highest-counted character…
I resubbed early and worked through carpal tunnel just to get those anniversary mounts. I am NOT happy my pain was wasted.
Yep… I was 383 and I added Squeakers and the Drake of the Four Winds and still showing 383
Insane. Why are you ignoring this thread, Blizzard? At least acknowledge the problem.
I am having the same issue as well. I learned a mount on my pally, it upped the total to what it should of been, learned a new mount on my main, a hunter and it did not change the total for lord of the reins. It is wrong if we have to learn it on a certain class to get it to count. My main is my hunter, not my pally. I shouldn’t have to worry about getting mounts on my hunter because if i get them on her they don’t " end up counting". I do not play my pally all that much. Blizz really needs to fix this. I even tried the whole log onto the pally and even ride around on the mount and it still has not changed. i am 3 mounts away from the achievement and I will be beyond upset, if I do not get it when i get the 300th mount because it may not be on my pally at the time.
Still no acknowledgement from the powers that be about this I see. Got the quest in Uldum today on my main that brought her total usable mount count to 384 and the achievement count went up to 384.
Logged onto my pally and her usable mount count went from 386 to 387 but the achievement count still remains at 384.
Yep, it’s still happening. My DH has her mount count at 251 so I should have had the 251 mount count ding, but I’m still showing 248 mounts…
Still happening. I just bought two mounts off the store, when I was 1 away from the 400 achievement. Neither brought my count up, and I also logged into all my other characters.
Probably because they won’t bother fixing this bug until at least patch 9.0.1, unfortunately.
So, my count did move, unevenly though… I was at 395, bought 3 frogs in nazmir, got the alpaca in ulduum, got the swarmer in ulduum… should be 400… nope… 399… A couple days later, I bought the alabaster mounts from the shop… still 399… I checked warcraftmounts dot com - my pally shows 402, my DK 401, my DH 400… and yet I sit at 399
And I’ve sent 2 bug reports, one before and one after spending actual $$ on a mount.
This has been an issue since 8.2.5 and the only way around it seams to be to get your new mount on the character with the highest mount count. Not sure what they got going on that makes this so bad