Ranger – “Defense” [Campaign 5, Mission 3]

What a nightmare. Two quests have been completed (i.e., the checkboxes are already checked): the city has been recaptured, and the heroes have been freed (by the way, thank you, VshNshd, for the helpful advice). All three heroes have been maxed out, but the purple one is still running around. It's been almost a year. Should I start over? Could it have been affected by the fact that after the garrison was recruited, some troops were taken from the city?