Locations:

Lower Gyfin

Mirumoks

To Trigger Bug Behavior:

0. Begin at a Totem (spawner)

1. Engage in Combat normally

2. Fight for an indefinite period of time. Usually less than 15 minutes.

4. While moving from one Totem (spawner) to another a far away Totem, impossible to agro, may be discovered to have Agro'd.

- From this point on all totems nearby will either explode (Mirumoks), not engage (Gyfin), or in some rare cases apparently reset.

Experience with this bug:

I first noticed this at Lower Gyfin. I would kill a Purple, Red, or Blue type creature-set. However, even if the creature-set indicated to kill the one nearest me a set far away would engage. As I play Tamer, I figure thought this was Heilang. However, I turned him off and it still occurred. 

Later, I moved to Mirumoks. The results were the same. Heilang was turned off, but as I would finish one Totem (spawner) I would go to the next only to find it wasn't there. Or, it would explode on one-hit. 

I finally realized this is a bug rather than intended behavior when creatures from the furthest Totem away from me were running towards me only to de-agro some tens of meters away. Usually causing the Totem to explode, reset, or go into some weird stand-by mode. I'm not even sure this is a stand-by mode. However, in the case of Lower-Gyfin it would just continue to spawn minor waves of enemies. Not enough to be impactful. Neither benefically nor detrimentally. It simply wouldn't "leave" where it agro'd from unless approached. 

Lower Gyfin may have other complications. 

For instance, I noticed since this bug that the Butcher spawns rather slowly at times. That is, the creature spawn rate will cease or diminish. The major colored triune types to be killed to continue to receive the kill-buff will not engage. Then the Butcher, after about a minute or so, will finally appear. This leads me to think there is some kind of relationship between the Butcher's spawn rare and this bug. It probably isn't very helpful knowledge on a player's end, but I'm pointing it out should it help you.

There appears to be no work around for this at the moment. I'm going to move on from grinding Mirumoks for now as it is pretty thoroughly game breaking. Lower Gyfin is just inconvenient. 

Thanks

Just to check, you said you turned off heilang, but do you perhaps have a pet with aggro ability? because that happens when the pet aggros mobs, and it often is not visible