https://wccftech.com/xbox-series-s-backward-compatible-one-x/

The Xbox Series S may not be able to run older Xbox games with the same enhancements seen on Xbox One X, according to a recent analysis.

According to Digital Foundry's John Linneman, the less powerful next-gen console will not be able to play original Xbox and Xbox 360 titles with the enhancements available on Xbox One X, like 4K resolution support, mostly due to the available RAM, as the Xbox Series X has 8 GB of available RAM as opposed to the Xbox One X 9GB. As such, the Series S is likely going to play backward compatible games in Xbox One S mode.