So keep with me on this, let's start it out simple we all know what the Warp in the West was but take this into mind, what if one of the events of the Warp in the West was keeping the Mantella intact on Mundus instead of it being sent to Aetherius or the Mantella was sent into Aetherius and there is a Mantella on Mundus so is it possible that after the Dragon break the Mantella exists in one way or another on Tamriel i.e. meaning two Mantellas exist, we just don't know where it currently is?

IIRC, every ending ultimately results in the Underking reclaiming the Mantella anyways.