The S rank spawned. Let's say Agrippa. Someone popped an Alexandrite map for the sake of Alexandrite and went on their merry way. 10 minutes have passed before someone came to look for Agrippa. He is now without that immunity. What do?
Similarly, Ixion effectively can be pre-pulled, the person who found him just has to have the ash. It's not that hard to do even on your lonesome.

I agree with the notion, but this won't solve it. Only thing we really can hope for is SE either revising the ToS regarding hunts (unlikely imo, nor should they, we're not obligated to share these things), or doing something else like increasing spawn rates, at least for ARR hunts. It won't help with these guys but it'll open up more opportunities.