So I noticed that during the "In The Deep" mission you can stop to kill the enemies while trying to escape from the catacombs. Most of them just respawn, but the wizards don't. When they die, there are 2 big knight bosses that appear. When they die, a bunch of boss shriekers appear all over the map. I killed all of them, and the enemies stopped respawning in the first area. I don't know if this is the beginning of a secret or something, but I think it might be worth looking into, especially if it is the replayable mission of the week.