These two encounters are not doing the secondary effect: Vampyric Embrace (which does dmg is not returning 50% as temp HP to caster) and Killing Flames ( Which does it's dmg is not giving the HP to caster also .
They do appear to WAI in open world combat but fail the HP returns in quest driven instances.
It is failing in the wyllowwood battle against Acturia in the entire instance and the Quest driven boss battles in the expeditions going through the twisted gate and gate riddle.
I might mention on a side note: The mimic boxes in both twisted gate and gate riddle are totally immune to all SW dmg as well. Necro pet, companion and every power (at-will, encounter, daily) including artifact don't even dent it. You can't get runic remains or gate as you remain taged as in combat. Only solution I have found is die and respawn at campfire and don't try and loot it again.