My information is based on datamined information from near the beginning of the game. Those posts all say the same thing. It’s tenacity minus potency. There is nothing added to potency. The only modifier in the calc is the min 15% chance. It seems like the reason the 100% potency idea comes up is because people try to explain the mechanisms as if the debuff is checking to see if it lands. Basically from the debuffs point of view. My understanding is that the check is from the characters point of view. Can it avoid the debuff. From that point of view, there is only one tenacity check. And coding wise it’s very simple to set a min value to any check, so it doesn’t require two checks. It’s one calculation to get the value needed and then a “roll” to see if it’s successfully avoided.
All of this is second hand info though. I did none of the research and am just repeating what others who did the research said. There’s an article on https://gaming-fans.com/2017/05/understanding-potency-tenacity/#:~:text=PsychoPoet,%20author%20of%20our%20popular%20speed%20optimization,%20offense/CC/CD,