So it appears that activating Mayhem mode in TVHM is a bug. You’re not supposed to be able to do it. That means we are meant to play through the story in TVHM with zero mayhem difficulty modifiers.
That playthrough was incredibly easy for me to run through (I’m on console so I was already forced to do it without Mayhem). I felt at least challenged in BL2 and TPS in comparison. Even BL1 felt harder in TVHM.
If non-Mayhem TVHM is the intended difficulty, something went terribly wrong when developing the challenge level.