Quoted from VG Tech on their Gears 5 tech test:

Xbox One X in the Versus mode uses a dynamic resolution with the lowest native resolution found being approximately 2560×1440 and the highest native resolution found being 3840×2160. A form of temporal reconstruction is used that can increase the pixel count to 3840×2160 when the native resolution is below this figure. A native resolution of 3840×2160 seems to be rare here but pixel counts at 3840×2160 are common due to the temporal reconstruction.

Xbox One X in the Boot Camp mode uses a dynamic resolution with the lowest native resolution found being 1920×1080 and the highest native resolution found being approximately 3200×1800. A form of temporal reconstruction is used that can increase the pixel count to 3840×2160. A native resolution of 1920×1080 seems to be rare in this mode and pixel counts at 3840×2160 are common due to the temporal reconstruction.

Xbox One in the Versus mode uses a dynamic resolution with the lowest native resolution found being 1280×720 and the highest native resolution found being approximately 1600×900. A form of temporal reconstruction is used that can increase the pixel count to 1920×1080. Pixel counts at 1920×1080 are common here due to the temporal reconstruction.

Xbox One in the Boot Camp mode uses a dynamic resolution with the lowest native resolution found being 1280×720 and the highest native resolution found being 1920×1080. A form of temporal reconstruction is used that can increase the pixel count to 1920×1080 when the native resolution is below this figure. A native resolution of 1280×720 seems to be rare in this mode and pixel counts at 1920×1080 are common.