It's very possible that the chance to resist formula was changed in the last update, without any mention of it in the notes. A recent datamine has found that the chance to resist as per the game files is
Chance to Resist = max( 0, Tenacity - Potency + LevelDelta ) + BaseUnitResistChance
This is much different than what devs previously claimed it was, which was
Chance to Resist = max( 15%, Tenacity - Potency )
Where I'd assume that 15% is the BaseUnitResistChance. If this is indeed the case, then the level 90 rancor now has the potential for an extra 10% chance to resist from the leveldelta, as well as the tenacity and potency difference. In other words, if this is true, then some players may need as much as 25% more potency to achieve the same tmr proc rates as they did before the update.
Source:
https://www.reddit.com/r/SWGalaxyOfHeroes/comments/597p32/code_mined_chance_to_resist_negative_effects/