Tipped Arrow of Harming acting strangely

Fully charged arrow shot with Harming deals more damage than normal arrow (working fine here). EDIT: this is wrong, damage is the same with or without harming, damage is whichever is highest of the two.

Fully charged arrow shot with Harming II and bow enchanted Power V deals the same damage as normal shot with Power V (was expecting additive stacked damage, although might be intentional if the damage was too insane).

Fully charged Harming II shot deals less damage to a player in full diamond armour than a shot with no charge! (expected fully charged shots to always deal more damage than zero charge shots!).