Does the fight involve cipher spells like desintegrate? I don't believe vithraks do this, but maybe Sissak himself does those things. Anyway, i have tested the charm effect to death thing more thoroughly now by using summoned wurms and eder as guinea pigs, it happens in 90% of the case when the effect expires the affected character simply dies, sofar only spores in sanctum does this, normal dank spores is not a problem.
I also have encountered this issue sanctum. It seems to be related to the charm effect, whenever someone get charmed by those giant spores and the effect expires the affected charecter is killed off. On a different note those little sporlings in sanctum seem to remove wizard buffs with their dominating spores, went in full buffed including a antidot and the first attack from sporling dispelled everything including the antidot. Very frustrated and made me register in forum and make this post. Will there be anymore patches? Because those issues seem pretty game breaking to me.