Huh I don’t know what’s going on but I just got the Chrono Corsair. I was just getting some of my level 70s the buff and my main their 5 quest done then bam! Achievement, mount put into my collection.
Not sure if this is intended or if it’s going to be fixed today, tomorrow or maybe next week since today is a federal holiday.
Edit: to clarify what happened. I did the 5 quest weekly on my main, then I got my level 78 and 79 to 80. All three of these characters ran at least 5 dungeons. Then I switched to my level 70 paladin and got her the 5 dungeon buff then switched to my 70 evoker. As soon as she got the 5 dungeon buff is when the achievement popped.