I found this thread on the LM board - could be old news, but it was new info to me. Seems that healing threat only affects a mob if you heal the target of the mob, or a fellow the mob has damaged with an AoE.
I have a hard time believing this but will test later. just to make sure.
take the barrow downs skirmish for example. kite for ~40m healing someone who does not have aggro.
if the LM throws heals on me every 20s, we'll say 30s to be conservative, that's about 80 heals. that's a lot of healing. and seeing as I am the only person with aggro on anything (excluding maybe a lieutenant they're burning down), why does the aggro never snap to him? I am constantly healing the tank, who has aggro on 0-1 mobs, to hold aggro on everything else. How do I keep the aggro if not through healing? Doesn't equate.
I also notice that the two tests as of a couple hours ago were done on pets. Pets have a history of having a bugged relationship with random things...shield of the dunedain 100% bubble, turtle aggro exploit, etc. this could be one of them.
Shock and Awe
Hydra - Captain. Kraken - Guard. Ra - Warg