This is a lot of fun. iOS 13 is using ARKit to make your eyes look at the person to whom you are connected via FaceTime, rather than at the actual slight angle as you look at the screen rather than the camera.
To see this at work, check out this bit of video from Dave Schukin. In the video, keep your eyes on the eyeglasses arm, rather than on the eyes, watching for that arm to warp as ARKit does its thing.
If you have the iOS 13 beta, you can enable this bit of trickery in Settings > FaceTime, but only if you have one of the most recent model iPhones.