Anyways. I did a search and did not see anyone mention this bug that I could find.
I generally build my base near Clownsteady between the two Cannibal camps so that I have a steady stream of good combat for skill gains. I noticed this bug originally and thought it might have been due to a memory issue when I was playing around with the squad size multiplier in a previous save since I never had this happen before that one time or after.
Basically, the bug happens when you're engaged with cannibal's holding a prisoner and when they die the prisoner which is not unconscious or in a recovery coma is already on their feed and ready to fight. Now that sounds fine, except for one problem. You cannot attack them. You can defend against their attacks and if you hit pause and try to select them it is if they are not there but they are fully capable of dealing damage to you. Ctrl+Shift F11 does not fix this issue as it isn't a navmesh issue that I can tell.
So far the only way I have been able to fix it is to save the game, close the entire client down as trying to just reload it without restarting the client does not correct the bug and when you reload from a fresh start the bandit is now capable of being damaged again.
Something tells me this is an issue with a memory leak if it is only resolved restarting the client and reloading the game after.
Hope this helps.