Funny thing, a dev clarified what certain terms mean and how they function. What Rending does (and Brittleness by extension) is change a percent of the damage mod to be unmodified rather than use the flak / carapace damage mod, which you can see by hovering over Penetration or Stopping Power for most weapons. If the flak or carapace armor mod happens to be more than 100%, well, turns out you’ll actually deal less damage once Rending or Brittleness gets applied!
I don’t have a Thunder Hammer to check, but I do have a Combat Axe and Chain Axe, and yup, both those weapons can have their flak armor damage mod go more than 100%, so you’ll see reduced damage. Technically working as intended, I suppose. Even then, probably not the best way to go about it - hopefully it’ll get looked at.