I had heard about it. Never believed it. Till now. It’s happened twice to me just recenly. Both the Grand Black War Mammoth and one of the Primordial Direhorns didn’t count. I’m at 399/400 for Horde of Hoofbeats but…nada. Neither one counted. Yes I logged onto all my toons. Yes, that includes my pally and tailor mage. No, it didn’t make a difference. SMH.
Please check my answer here Achievement not updating mount count, might explain your issue.
I read your explanation. I get what you’re saying about characters with counts and hence why I mentioned logging onto my pally and mage. The fact remains that I got at least 2 mounts within the last couple days. They are not “exclusive” mounts. In fact it started with Xinlao. So that’s at least 3 mounts. None are “exclusive” to a class or faction. All should have counted towards the achievement after logging onto my character that has the most mounts unlocked. There is zero reason for these 3 mounts to not have counted, yet they did not.
Additionally, i’ve seen posts on reddit regarding this bug by people who are well aware of the logic you described in your explanation. This seems to be a fairly common bug unfortunately.
I’m having the same problem. I got my 200 mount achievement on Friday, and with the reward from that I was at 201. I might have gotten another mount that day also (I can’t remember the order I got them in).
Yesterday I got the two mounts from the Sons of Hodir, and my mount count is still sitting at 202. I’ve signed onto all of my characters. I even bought a store mount today to see if that increased the count and it didn’t. Still sitting at 202.
My main is a pally and she definitely has the most mounts of all of my toons, so this isn’t an issue with not signing in on the right character.