Yes, I also experienced this when I roll as a WHM. As a WHM, your ohshtbtn is kinda limited, which is Benediction. Divine Seal can manage a bit, but after that the WAR will burn out their hp again. Then switch to Presence of Mind for a faster cast speed. Usually I just switch between these spells one by one. Coordinate with the WAR to pop some CDs if the fight is going to take a bit long, because you cannot afford to burn out your mp fast.

(Unrelated side note: SCH is a better warrior healer imo, because lustrate can help much to match heal WAR huge hp)

Well, WAR will need to adjust their play style with the new adjustment, so give them our support as a healer!