Normally, healing threat should be evenly split between all enemies in combat, to account for increased damage taken from all those enemies etc.
However, there seems to be an issue when fighting multiple packs. I pulled a pack of 3 mobs, then one extra mob joined the fight. After some time of tanking those mobs & healing my warrior, the priest had 4k threat on each of the 3 mobs in the pack, but 12k threat on the extra mob. Apparently the game doesn’t recognize those mobs as part of the same combat scenario and doesn’t split healing threat accordingly.
This can cause real issues in dungeons when healer is forced to heal a lot and rips aggro on mobs that are somehow counted separately.
Not sure if related, but I have a similar issue in Ramparts, normal, on the multistage dragon boss.
When healing as a druid, I don’t think I ever pulled aggro .
When tanking, it’s usually fine.
When tanking with a priest healer, quite often when the dragon spawns, it runs for the priest despite getting Avenger’s Shield. So I learned not to use AS on the orc, simply approach it and use Consecration + Holy Shield, but that takes a few seconds to take aggro and I usually have to taunt back. Regardless, I lose aggro once it expires, taunt again as soon as I can, priest alive or not, and then the dragon comes, and runs for the priest.
I don’t know whether this is a complex issue, such as a warrior stunning the boss (shouldn’t) and my aggro potential being 105% anyway and with losing initial aggro, it’s literally mission impossible (without Retribution Aura and Holy Shield proccing, as I’m not being hit, all the while the priest is spamming heals - and not using Fade).
(Unfortunately without a shaman present, the only viable seal is wisdom, which makes it a tiny bit more difficult as I don’t have regular single target holy damage. But I tried that, too, maybe works even, just not enough runs to sink in.)
(One silly possibility is that someone damages the dragon while flying around, which causes it to accumulate aggro. Which might relate to your problem, the mob accumulating threat despite not being pulled. I’m not sure where the threat add-on takes its numbers from, back in the olden days it used to be calculated client side.)
Priests have 1/2 healing to threat conversion, while paladins have 1/4. I’m not sure if druids or shamans have different values, but it’s clear that priests were designed to have more threat issues than at least paladins. I’m not sure if it’s different from the issue you’re describing, but at least this part is somewhat documented and “working as intended”.
I haven’t been able to find anything describing the broken threat split mechanic though - all sources simply say it’s supposed to be spread between every enemy in combat, but doesn’t specify what counts as “combat”. Maybe it is intentional, but then it would be very hard for a tank to compete with 1k+ potential TPS per target that a priest spamming max rank greater heal can generate - not to mention AOE heals.
This specific issue was observed in shadow lab, when we got ambushed by a rogue while clearing trash before second boss. But I also noticed severe threat issues while questing with a friend, even if he got every mob in the pack to half HP, a couple heals could pull all the mobs to my priest.
Just noticed it’s super broken in black morass - every extra mob that spawns from portals is counted as a separate combat instance or w/e, so healer threat is copied for all of them instead of divided. That explains why it sometimes feels impossible to get mobs off of healers in there. There’s no way a warrior can compete with a priest casually generating 5k threat on every mob with a single prayer of healing.
