You had already killed their first troop is my guess, so their hero was technically in first place.
They matched skulls, which triggers the Assassinate trait killing your last troops AND triggering the Assassin Class talent ‘Shadow’s Call’ hence the summoned in Sister of Shadow.
Other notes: To support this the Sister of Shadows has taken NO burn damage unlike the hero and Ishbaala, Magnus is Impervious so that is my he is undamaged.
I maybe wrong, but looking at the evidence presented so far, this is my conclusion. So, not a bug.
Maybe it was a displaybug and the shadow sister was already dead from a instantkill trough my hero… (dragonhero = triple skulldmg vs burning enemies) but i didn’t notice a skull before and take, with this combo, every skull for the instagib…
But that would explain whats happened.