I haven't been able to initiate the Treasures of Ruin quest either, and I'm on tier 4, having just finished the Victini quest. I wouldn't expect legendary quests to become locked on later tiers because if that was the case any new content/legendary quests would be completely inaccessible to players who were past that point. But either way, since neither of us seem to be able to initiate Elias's quest, I'm guessing it's bugged.