I have 295 mounts that apply to Lord of the Reins, 300 mount achievement.
I buy Reins of the Thundering Ruby Cloud Serpent from AH and use it.
Get popup that I now have the new Serpent mount.
I ride the mount.
I check Lord of the Reins and it still shows 295 mounts.
Cloud Serpents no longer require Cloud Serpent Riding even though my paladin i’m using for this achievement did earn it back when it applied.
Can a Dev give credit for the new mount?
Have you checked the achievement count on all of your characters to ensure you are getting proper credit for all your mounts?
Yes. I checked all my characters regardless of class or level, horde or alliance, and Lord of the Reins in 295 for all.
These type of achievements used to show the count for the character you were logged in on at the time, my paladin and warlock would show a higher number than my mage. But they must have changed that now.
edit. not sure why my little monk was showing instead of my paladin in first post…
From reading other posts, it does seem like some wonkiness is occurring in regards to mount counts.
Please make a bug report so that QA can be made aware of the issue with the achievement not reflecting the correct counts.
You can do that in-game (ESC-HELP-SUBMIT BUG) or you can make a post in the Bug Report Forum.
Hopefully, Blizzard can get it sorted soon.
Thank you very much 
You’re welcome.
Best of luck.
I’m in the same boat of trying to understand what makes the mount tracker progress. I was sitting at 327. I finished Glory of the Legion Hero and Glory of the Argus Raider. Both give mounts. My counter stayed at 327, even after logging in and out of each toon.
Today, I acquired the Headless Horseman’s Mount, and it went up to 328. I’ve had this happen before, where I’ve seen the mount counter jump up 2-4 at a time. I’m wondering if it’s just buggy and won’t count again until my Pally gets a new mount… I presume he has the highest of my toons. I’m thinking once he personally gets a mount, it will recount his mounts and jump up. Like it only recounts your mounts on the toon you acquire a new mount on.
But, just a theory…
I got my Ruby on my horde warlock at the ah, because she had the gold to buy it and I couldn’t send the gold to my alliance pally so I thought just buy it and it should be available to all so it won’t matter…
I think my pally has one more mount open to it from the crusader area in wrath than my lock does. So I’m hoping the Ruby made my warlock count go up one to tie my pally and I will make sure to get the next one on my pally then Lord will recount and add 2.
Ok, it’s fixed.
I acquired the Crimson Tidestallion just now with my Paladin and the count for Lord of the Reins went from 295 to 297, where it should be by now counting the Ruby and Crimson.
So the updating of mount total achievements occurs only when a new mount is added and then only for the usable mounts for that character that used the item or just gained the achievement to get the mount.
It does not update the count to show the correct value for the character we have logged in at the moment, the count is set no matter which character looks at the achievement.
This used to, many years ago, show the correct value for the currently logged in character.
So one way to get around this is to acquire an item that grants a mount but only use it on your character with the most mounts.
Or only do mount granting achievements on your character with the most mounts.
When a new mount is acquired a small howmanyusablemounts subroutine is called and a total is given. Why not just run that each time a character looks at this achievement instead of only when we get a new mount.
Then this problem and confusion would not happen. It would not matter which character got the mount or how they got it. Each character would see an accurate number for them.
The only drawback may be that someone checks this on a low mount character and thinks that number is the highest because they forget that another character has more. Though I doubt someone seriously trying to get this achievement will make this mistake.
The best solution would be for Lord to remember the current high count whether the character with the highest is logged in or not and update the count appropriately.
Hope this helps others that find themselves in the same frustration I was in.
Sounds about like I thought (I was the DK above. Idk why it was on that toon). Have to try and get mounts on your toon with the most mount count for it to do a recount.
Bummer. Dungeon mount farming on a slow pally can be rough since they’re so slow. But, I guess it only matters if getting the mount count would actually finish off your next achieve. For me, going from 327 to 330 doesn’t really matter. But as you inch closer to the next mile stone, it starts to matter more.