The ring is held by the last rat of the popup batch. In CF, the last creature of same mob doesn't repop unless all the identical creatures are dead. It's the same phenomenon as arial guards stacking or elite guard with the key to midnight dragon not popping up. I believe that it is caused by the game only checking what mobs to pop and what amount, then generating the missing amount of mobs, starting from mob #1 of that type and continuing until the number of mobs missing has been repopped.