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After completing the main game and it's dlc for 100% I returned to Regan's location in northern East Anglia and looked around the area. When I walked outside the area I still got the message "You are leaving the fight area" as if i'm leaving the area whilst the fight is going on.
Regan has been killed hundreds of hours ago, there is no fight going on. But the message constantly pops up if you take the route I'm walking in the video below, the path to the right of the strung corpse you'd normally interact with to spawn her.