Towards the middle of the quest line in Lincolnshire, enemies no longer attack Eivor. The character can roam freely and assassinate any hostile NPC without triggering an alert. In the final mission of the quest, the raid on Anecastre, I ran through the entire mission, and even stood next to a Goliath, without taking any damage or being attacked. I entered the cathedral where my target, Bishop Herefrith, was waiting to be assassinated. I stood next to him and ten other soldiers without conflict.
I’m not 100% sure, but I believe this bug began after doing a world event near one of the ponds, and I was being attacked by an elk while in a cutscene following completion of the event. The event had me enter a camp next to a pond. A Dane and his brother were being held captive. I had to free one of them while the other was sleeping next to him. Then I had to carry the sleeping brother while following the other around the entire pond to safety. Once I set the sleeping brother down the cutscene began, and I was then attacked by the elk two times. Afterwards, a group of soldiers ran up to us, and I was able to assassinate all 5? of them back to back.
Also, after voting for the next ealdorman, when the Bishop locks the room with all the soldiers and NPC’s, I was able to assassinate all of them without harm.
Another possible instance where the bug could have started, was when I had to find and speak with Acha after Hunwald found out she lied about his father. At the edge of Lincoln two guards stood where I had to interact with her. I alerted the guards as I began speaking with her. At the end of the conversation I called my horse and left the area, so I cannot verify if it was at this point when the bug initiated.