According to The Verge, the Smart HDR feature present on both phones essentially chose the wrong base frame for HDR processing. So, whenever iPhone XS or XR users took selfies, the camera would sometimes opt to capture the moment with a longer shutter speed. Instead of appropriating a shorter shutter speed to capture more details.
Another reason for this phenomenon is the lack of OIS on the front-facing camera, which results in a loss of detail when combined with the long shutter speed quirk. iOS 12.1 is still currently in beta, but the fix should ensure that the phones will make their HDR camera system pick the sharpest base whenever it is used to take selfies. (Source: The Verge)
