I've seen it a couple of times actually. But no clue to be honest, haven't seen the script - maybe there is multiple outcomes, multiple possible scripts or loose scripts, that the NPC may or may not do that, considering that they can't kill each other past 85% anyway. Or just bugs, always an option.
Is there really such a need to desperately figure out an answer that is different from the most obvious and likely scenario - the NPC bugging as many others have reported it to often do?
She was indeed stuck on the ramp fighting a figment. And I was stuck in combat because of it for a while and that did quite a bit of damage to my run. BUT I noticed it at one point, went back and killed it, then watched as she comes back with me to the Red Boss. We fought it TOGETHER. And THEN she went to the third boss to fight it alone X)
I've been doing multiple visions every week since 8.3 release. Got stuck in combat a fair number of times, but every time that was for a good reason that I could identify.
Here's the thing:
If there was a major bug affecting content such as visions, be it people getting stuck in combat for no reason, or NPCs pulling bosses, people would be so outspoken about it on community sites that it would get fixed within 3 days. The fact that it is not so, means that most probably if you are experiencing such issues it's human (yours) error, rather than a general problem with this piece of content.