Boss marked as done after killing one out of two mobs

I engaged with the Orok Campfire on Act 3 Sandswept Marsh. They are two monsters and there's an icon in the map for them.
While fighting them, I killed the bigger one and died for a purple explosion made by the skinner. After respawning in the check point in front of the camp, they no longer are present, there's a Lesse Jeweller's Orb on the ground (where I died) and the icon in the map is grey, as If I had killed them.