Yesterday I finally got the 500 mounts achievement that i’ve been working on for months. Today, it’s gone. It says “achievement in progress…” but it shows 500/500 ??
I’ve never before lost an achievement. Wtf is this ?
I can still use the otherworldly otter mount that was the reward for this achievement btw. But it shows as I dont have the achievement…
I don’t know but you should submit it to bug report because this is not normal. Gz on your 500 mounts! Still running 30 times a week sepulcher for that god damn mount to get my 300 achievement 
Edit : if you have a level 10 on NA post there, there is a higher chance your issue will be looked into.
I’ve stopped submitting bugs. Blizzard never fixed anything i’ve reported, ever.
Yeah I feel you but that’s the best I could tell
Yeah, i hear you.
On my alt, I still have the achievement (not a dk or pala) and on armory, it says I got the achievement. Strange.
probably a bug .report it in game .i had reported few bugs last month and they fixed them in last maintenance .so hopefully yours will get fix too soon
Before doing bug reports etc, can you try resetting the UI
It could just be a display thing - let us know if that works 
What´s most likely the case hier is that one of your toons (but not the one you´re playing on) qualified for the achieve which is account wide, due to faction /class /profession mounts pushing them over the 500 mount threshold that the char you are currently on has not yet reached because he cannot access those mounts
For ex a Paladin has 3 order hall chargers while a mage only has the one disc, but more importantly also the original charger and warhorse which no other class can summon… add on for ex. flying carpets from tailoring, and the pala then has a total of 7 or 8 mounts the mage cant use. So the pala could get the achievement, and unlock it for the account, wheras on the mage it would show as 500/500 not completed because, while being unlocked for the account, the mage himself is still 7 mounts away from qualifying for it. If you mouse over the points shield on the right, it should show you the toon that first qualified. 
Grats to the Ottuk… I´m waiting to see what comes next @600 /650 /700 or wherever thhey put the next one…
No unfortunately, it looks the same
The bugs in this expansion and patch are limitless.
I can’t even remember how many of them i have seen, probably not a single day without a bug.
I won’t be surprised that players lose all their achievement one day.
No that isnt it. The achievement popped on my main (hunter). I’ve since obtained 4 new mounts (on a roll today) and there’s been no change.
Well, checking your armory it does show as completed. What does it look like on your hunter?
PS: I just realized that a lot of the mount collecting achievements are shown as not completed in-game! For me, 100+150+200+250 are “un-earned” - although the rest up to 500 is marked as done…
While not a mount but I have completed teir sets and they show not complete lol yes I did open report bug but I doubt anyone reads them lol.
In that case, I have no reasonable explanation to offer, only mediocre comedy relief…
DId you spend too much time in Azj’kahet and the bugs followed you home and settled in your mount collection? Have you tried shaking it really hard to get them out?