I was testing various Deal Damage on Parry gems/affix on shields and i’ve noticed big issue.
Gems on white pieces of gear display incorrect(smaller) values - after enchanting the values displayed are correct. Also there is no fixed number by which they are diminished. Sometimes it shows 7 on white piece when 10 is true value that is shown after enchanting. Sometimes it shows 7 and then 16. But they deal the same amount of damage(before and after enchanting) in combat.
I believe same issue is present on Damage on Parry mod that can be rolled on shields. For example my shield shows 6 Plague damage but in combat it deals 19.