There are a few discussions regarding this potential issue, I don’t have one of the new devices so can’t compare firsthand but from what I’ve read it could have something to do with how Smart HDR works.
Apparently SHDR takes images at a higher ISO and then combines to get the photo, this blending might not completely get rid of the grain especially in poor light, hence the difference with the X.
Whether the above is true or not, you can test by going into your camera settings and turn off Smart HDR (again don’t have device to test but believe it can be done).
I believe Apple released an update to correct over smoothing awhile ago, may be it’s gone too far, if so they may resolve in future updates, but until that arrives I would try messing around with the Camera App Settings and see what you get. The Xs family definitely have the potential for great photo’s though and in good conditions definitely blows away the X (which I still have).