I've noticed this issue a couple of times on my hunter and last night during Crota. This happens to me during VOG which is why I don't touch the shield anymore. When I melee praetorians with the shield and their HP gets low I go to smack them for the final hit and BAM they restore health in the blink of an eye after my hit makes contact. Also [spoiler]when fighting the gate keeper with the sword when I got his hp down to a sliver I went to get the final hit and his health came back when I hit him and he killed me[/spoiler]