The first Harbinger I fought was the Harbinger of Defilement. I then went to do the Black Sun, but instead of fighting the Harbinger of Hatred, it’s the Harbinger of Defilement again. I tried doing the Stolen Lance, and I fought the Harbinger of Pride. Went back to Black Sun, and it’s Defilement again. It seems like I’ve somehow changed which harbinger shows up for Black Sun specifically, and it won’t reset.
All that being said, defeating the Harbinger of Defilement in Black Sun still counts as a win against the Harbinger of Hatred, so I’m not prevented from moving onward. I’m still upset that there’s a boss I am incapable of fighting with my disintegration mage, though.