Sadly, this is most likely NOT a bug.
This is more emblematic of Blizzards design tendency towards wankery.
It’s not enough that you spent many hours of your time completing the 159 OTHER achievements for the mount. NOW you have to log in every day (sorry if you have an outside life) in the tiny hope that you catch the ONE DAY out of months that this might actually be available.
If you miss it? Well… hope you didn’t have any plans for the summer.
And remember, this FOMO nightmare was brought to you by Blizzard, the company that “respects player time”. Yeah, RIGHT.
I understand I’m coming on quite bitter and angry. But I’ve also been through this twice before. I did “Back from Beyond” and “A World Awoken” - completion on both of those were held up for MONTHS by bugs Blizzard utterly refused to fix until TWW’s release. Despite having touted both of those achievements, as a GREAT way to pass the time between expacs.
So this situation is regrettably par for the crappy course, to me.
I’m assuming they’ll go to Legion after this, so it might be prudent to figure which events go MONTHS and MONTHS between repetition. Because they will CERTAINLY be required.