Infinite abyss pit

Was running a sanctuary map and came across an abyss that led to a stygian spire, but after killing the stygian spire the final abyss pit kept spawning monsters, I stayed on it for a few seconds but it just kept going so I continued on and killed the boss and came back to it still spawning monsters, not running abyss scarabs and none of the idols i'm running even have anything relating to abyss as far as i can tell.

Location of final abyss pit portal at the top, boss in bottom right:


Idol setup: