Sanctum - new lightning mirages spawning when in reward room

I was running a sanctum and I killed a rare enemy with lightning mirage, finished the room and went into the reward room, and then a new lightning mirage spawned on me while i was in the reward room

maybe this isnt a "bug" per se, but it seems like something that should not happen since the reward room is meant to be a safe place after finishing the main part of the room