So did you changed how true damage works? Currently true damage is not affected by any damage reduction nigher amplifier. https://leagueoflegends.fandom.com/wiki/True_damage
That is inconsistently applied across the game right now. I'll grab some examples (first that comes to mind is that the formerly named Ninja Tabis reduce Camille's 2nd Q Attack by 12%, despite being true damage), but if we make the call that we DO NOT amp true damage (I agree with it not being reduced btw), I'll come back and correct my post above later.