Details of Xbox One backward compatibility on Series S:
Xbox Series S runs the Xbox One S version of backward compatible games while applying improved texture filtering, higher and more consistent frame rates, faster load times and Auto HDR.
https://www.gamespew.com/2020/09/xbox-s ... -confirms/So as suggested, the 'X-Enhanced' features added to some games during the mid-gen refresh will be on Series X but not on Series S. Seems this is due to the Series S being designed as a lower res machine rather than aiming for 4K gaming.
Edit - the vast majority of the X-Enhanced games are 4K:
https://www.resetera.com/threads/xbox-o ... ils.24378/