When striking a target like Chen’s stagger, it will only apply the heal from Saintly Greatstaff. When striking Protected, it still applies 100% of the healing. Healing is not applied when striking Illidan who is using Evasion.
Additionally, 100% of armor is ignored for Saintly Greatstaff’s heal.
Forums do not allow me to post proof, but there is a matching post on reddit with proofs.