Treasure Chest In Arlathan Crater That Is Impossible To Reach

There is a treasure chest in Sanctum Halls, in Arthalan Crater that is impossible to reach. It shows up on the map, but I have literally been over every single inch of the area and it is not possible to reach it. Since it appears on the map, usually indicating a treasure of at least some reasonable value/importance, it makes no sense it is inaccessible.

Here is where the chest shows up on the map, I highlighted it with a red oval around it:

This is how the interior of Sanctum Halls appears:

Arthalan Crater is a location that is only temporarily accessible and your party cannot return to this location once it is complete. So, it isn't one of those "you gain access to it later" situations since you can never return.

I'm not sure if it was a chest that was meant to be placed somewhere and just got left in an inaccessible location, or if access to the chest was meant to be implemented and was never put in place, but, regardless, it is impossible to reach it currently.