Some old iPhone X cases may not fit the iPhone XS, after all
The new iPhone XS has absolutely the same dimensions as the outgoing iPhone X and third-party case makers are listing their X cases as compatible with the XS. Apple, however, doesn't and has removed most mentions of the X from its accessory website.
The hall sensors of the iPhone XS may not be placed the same as on the X
The other could be that the XS camera bump turned out to be slightly larger than what we have on the X, and the hall sensor has been moved around, too. The sensor detects when you use a folio-type case, and open and close its cover, lighting up the screen when you flip it. Reportedly, the iPhone X Leather Folio case doesn't reportedly work with the XS precisely because of that.
An iPhone XS case would fit loose around the X's camera, and vice versa
The iPhone XS' camera bump is 1.37mm taller, and cases for the phone fit rather loose around the iPhone X camera, as you can see in the picture here, while snug, skin-type X wrappers may have stretching issues around the XS camera, so word to the wise if you want to use your old case on your new XS.