Hello aseem209,
Welcome to the Apple Support Communities. I understand that there is a dark spot appearing on the display of your wife's iPhone XR. Good thinking trying to restore the iPhone.
Based on this information, you can begin isolating the issue by taking a screenshot of one of the screens where you can easily see the spot. Send the screenshot over to your iPhone and see if it's visible in the screenshot.
If it's not visible in the screenshot, the issue may be coming from the display itself. If this is the case, you'll want to Contact Apple Support for more assistance.
If the spot is visible in the screenshot, this is likely to be coming from an app or the iOS on the iPhone. If this is the case, does the spot change locations? Does it appear in all apps or only certain ones?
All the best.