Skyrim's resistance formula has a rounding error (or strange feature) that enforces a magnitude of at least 1. Poisons of Damage Lingering Health have a duration of 300 seconds and a low magnitude. This causes a fair poison to be equally effective as a diluted one against enemies with high poison resistance even though the fair poison is supposed to be three times more effective. Dragons, the most common victims of lingering poisons, have high poison resistance since 2.0 and are affected by this bug.
A simple fix is to multiply the magnitude of the lingering poisons by 5 and reduce the duration to 60 seconds. This ensures the potions work correctly for enemies with up to 80% poison resistance.