I like when killing the boss reveals the map

It feels nice. It feels right. It makes mapping fun. However it only happens on some maps, and whatever system is in place to reveal the rest of the content and rares is too complicated. I don't get it, and I don't want to. I want to kill the boss and then clean up and go to the next map.

It's connected the having all the checkpoints... It mostly happens when there's only two check points... One at the beginning, and one at the boss.... Is why you get a reveal after the boss. And I agree, it would be better if it was only tied to the boss. Some maps checkpoints are a lil wonky to find sometimes