Leveling Roulette

[Iscah]

If the only explanation to this situation is "leveling roulette picks a dungeon at random", which I doubt is the case...

Picking something at random is exactly what "roulette" implies.

If one of those Lv30 jobs is newly unlocked, your chance of getting Sastasha is one-in-five - or one-in-six if everyone has unlocked Halatali. (And if you don't get Sastasha, your chances of getting something else equally unwanted are pretty high.)

But there's also a decent chance of you being pulled into high-level content, if you're needed because someone directly queued for a higher dungeon you're eligible for. But those times aren't going to stick in your mind the same way.