I suspect this is related to the oftentimes irregular pathing. Server gives bad position data causing npc's to calculate wrong paths in some cases and in this case npc thinks it is outside proper range so it leashes back.
There is a patch on Wednesday that will remove his tether entirely. If he leashes again after the patch - then it will be worth reporting.
Just as a point of reference … you were building all 4 ballista before firing? We did it tonight with firing all 4 and didn't have an issue with tethering. But yeah hopefully the patch removal of the tether fixes this.
It leashed on us tonight too, although we only had brought it to earth once and got it to around 80 somewhere. No big deal and we plugged along and killed it but still annoying. Yes we had all 4 ballista fire and got the emote saying so.
