Island Expedition Locked Sacks Help Plz

I am running them mythic solo to try and get everything I missed previously. I keep running into where you killed the mobs around sacks of azerite, pretty sure there’s always a necromancer in the bunch, and when they all die a crowd of untargetable skeletons pops up and the sacks stay locked. Map still shows a “powerful enemy” skull, and you can kind of see it but not target it. I have ran into this over 20 times in the 130+ runs I’ve done the last few weeks. Anyone know what the heck is going on? How do I unlock the named and the sacks? Everything around the area is dead by this point? Thanks in advance!