Not sure if the OP solved this yet, but I think I may have the solution as nearly this exact same thing happened to me except with Cinder Drift EX. I was 100% sure I had taken the quest for it, and 100% sure I saw the big "Cinder Drift EX added to Duty Finder" pop up but I couldn't for the life of me find it either in my quest list or in the duty finder.

I did a little searching on what the requirements for the quest were just in case I had somehow dreamed the whole thing and noticed that the ilevel requirement to actually do Ruby EX was a couple of points higher than I was. I bumped myself up a couple more ilevels and, bam, there it was in the Duty Finder and on my quest tracker. Weird UI quirk/bug but it seems like you can accept Duties/Quests that you meet the XP Level requirement for but if you are not of the appropriate ilevel then for some reason you can't even see the Duty at all (and of course the quest giver will just act like they already gave the quest to you because, technically, they have).