Hey guys, I've been having a very specific bug since release (and up to the new DLC's release) and I think I've finally identified why it's happening.
Sometimes, when fighting enemies with charm abilities (such as Fampyrs) I've noticed that Chanter summons sometimes appear under the influence of the enemy for no apparent reason. After some testing, I think this only happens when your main character is himself under enemy influence (charmed/dominated) even though your Chanter is not :
As you can see on this screenshot, my Wurms are somehow fighting for the other side even though they were summoned by my Chanter (who wasn't charmed during the fight) because my main character (who isn't even a Chanter) is currently charmed.
It seems to me like Chanter summons are automatically assigned to whatever "faction" the main character is currently serving, even if the main character is not a Chanter and not the character who summoned the creatures. I have replicated this bug many times, with different parties/saves and against different foes, so it seems to be fairly straightforward.