Hey I figured it out. When you reset the quests on TVHM your game restarts after the intro movies and you are not in Mayhem mode. The trick is to rush your way through the main missions, ignoring side quests until you get onto Sanctuary. The Mayhem pillar thing is there regardless of your playthrough because it’s not a quest. You can then set your Mayhem mode and return to Pandora to do sidequests with a challenge.
It’s not ideal, and really there should be the option to restart the game at level 50 in TVHM with whatever Mayhem mode you ended it, but maybe they’ll patch it.