I encountered 2 Shadowguard Portalseers in the general environs of K’aresh. Their Mending Leech ability has too short a cooldown and/or restores too much health. With the pair healing themselves and the other, they were completely unkillable. If I interrupted one, the other would simply out heal my damage (674 ilvl, 4-piece S2 gear Holy Paladin).
The current state of Mending Leech is a bug and needs to be altered.