I never ever got Titan HM, so i'm going to guess it's working as intended.
It just takes whatever dungeons people are queue'd up for that arent complete, and tries to fit you in there.
My guess is that Titan HM is simply done a little more often then the others (still have yet to get it, maybe no one does them at the time of day i enter the DF)
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[XI] Surivere of Valefor
[XIV] Sir Surian Bedivere of Behemoth
http://na.finalfantasyxiv.com/lodestone/character/2401553/