Frequency: Always.

World: Midgardsormr

Class/Level: Scholar (60) and White Mage (60)

Party or Solo: Noticed it solo, but I guess it's irrelevant.

Race: Miqo'te F

Steps: Two healers at lvl60 with the exact same healing potency stat, with the same healing potency spell, but different by 101 determination. Both heal virtually the same amount of HP, despite the large difference in determination.
The high and low numbers differ by a statistical error (due to randomness), the average seems to slightly be higher for the higher determination character, but not even by half of the determination difference.

Checked by casting Cure/Physick on myself a bunch of times without Cleric Stance or any buffs on.

If this is working as intended, please consider this a report on a wrong description of the determination stat.