Viridian Wildwood.

I just completed a Viridian Wildwood map.

Give that I had a fairly juiced waystone, with extra Rares etc, I was disappointed when the map was completed by killing the map boss, but I could not continue the map to kill the rares I had not yet seen.

I did not die so why stop me from getting 100% completion of rares?

The map is on somewhat of a timer. The goal of the map isn't really to kill rares. The goal is to find as many omens as you can. Killing rares comes from uncovering as much of the map as possible while trying to find the altars with omens in them. Sometimes you will see a wisp. Stand on it for a second to add to your seeing distance or extend the map a bit.