Well, this is an odd software glitch. Certain Pixel 3 XL units are displaying a second, virtual notch on the screen of the device. This is completely unrelated to the real, physical notch on the screen which houses both front-facing cameras, as well as sensors. The glitch is already being addressed by Google and will be fixed via an upcoming software update.
Several users testing the Pixel 3 XL have come across this issue, including myself. The issue on my unit has been temporary, and reverted back to the correct format after restarting the phone, as others seem to have experienced. UrAvgConsumer came across the second mystery notch, as did a number of Reddit users, as well as a number of other people on Twitter.