I got to the part of the signals and stashes mission to defeat the orc guards at a milk molar stash. I got to the milk molar stash and killed the Orb Weaver Jr.'s Then the regular Orb Weaver got distracted by my ant and disappeared. Ever since the that Orb Weaver disappeared, I haven't been able to complete the quest. The marker still shows up there, but nothing spawns, and the quest shows as incomplete. I've tried loading up back-up saves but that hasn't worked.  

This same thing happened me as well

Same sort of thing but at the blueberry patch, a bee glitched under the ground and de spawned because I couldn’t kill it.

 

Same

Same thing happened to us. We are stuck on defeating the O.R.C guards at the milk molar stash. Had some crazy where both my husband and I, and our ants died and as we were "waiting for res" we saw one of the orb weaver jrs run off. We also reloaded and went back and the one orb weaver was back, but even after we killed it the quest is still incomplete.