Hello 79firebird,
Let's use the steps below to restart your iPhone:
1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
You can also find those steps in this resource: Restart your iPhone A restart can sometime help address odd behaviors.
Should that still persist, do you experience the same behavior when rotating your iPhone to landscape orientation?
If you test that portion of your screen using a different app, such as Calculator, do you experience a similar behavior?
Let us know, and we'll keep moving forward based on your results.