I have seen these issues in the past. There were 2 possible options.
1 hardware defect. This means you will need a screen replacement. The problem is that this hardware defect might become present when updating and having a non-original Apple screen installed. At that moment I have seen cases where the screen bricks and basically stays bricked no matter what. The downside is that you can't replace your screen at Apple anymore if it has a non-original part if I remember correctly.
2 The more likely one, is a software bug. In that case, I recommend the following.
- Press and quickly release the volume up button
- Press and quickly release the volume down button
- Press and hold the side button and hold until the phone turns off
- When the Apple logo appears, release the button and let the phone boot up