I achieved a Wonder Victory with the Riftborn on the same turn that the Umbral Choir (AI) achieved an Economic Victory. It gave me the victory screen that said multiple races achieved victory, and it unlocked the steam achievements for my first victory on Endless mode and my first Wonder Victory.  However, when I proceeded to the score screen, that showed the game as a loss, even though we completed victory conditions on the same turn (I even had the higher overall score)! The Empire Chronicle also listed that game as a defeat.  Are tie victories bugged or is there some tiebreaker mechanic that I don't know about that would account for the loss?

P.S. It also didn't show the victory cutscene for the Riftborn.