So Sarivene Songblade randomly generated on my map. I fought her and defeated her. I leave for about 10 minutes, and she has regenerated back on the map instead of the woodsman placeholder. I'm not sure if this is normal and if the Named are simply programmed to keep on re-appearing until the map is reset from being away for a certain amount of time or if this is an actual bug. (I did fight her the second time to look up her name. I didn't remember it since I hardly see her around.)

Edit: I went back by the island to try to find debris, and I saw the placeholder was there.

Night, it's just a random chance that they spawn instead of the placeholders. Sometimes they spawn several times in a row. ^-^