Hey there. Could you take a screenshot of the triumph? Unfortunately you've got everything hidden, so the API can't be checked from that side. Legendary Ascendant Explorer gets awarded once all three realms have been completed. Only Debris of Dreams and Ruins of Wrath have been made available on Legendary so far. Are you saying it is not ticking off Ruins of Wrath?
Completed fine for me last night did you kill all champions you encountered