Just to extinguish even the more improbable options, in addition to rolling a new character, I also tried going to the Sub-Arena without entering any of the other rooms in the Dig Site, as well as entering it before I talk to the people in the cage, but nothing worked.
I did notice though, that in both of my "playthroughs" the Sub-Arena was missing the appropriate yellow diamond-shaped map icon. Can't say if that's related or not, but worth mentioning, I suppose.