Stray Lines when Writing on iPad Pro

I use an iPad Pro (11-inch) (2nd generation) and an Apple Pencil (A2051), mostly on Goodnotes 6. After a fireside accident, I had to have the screen replaced and the body of the iPad flattened and straightened. There are very very subtle shadows on the screen.

While writing my notes or annotating PDFs, the 'ink' goes glitchy, creating straight lines and dots at a rate of hundreds per second. It is hard to describe, so I have uploaded a video to show how the problem manifests itself, and a second video to show - using undo - how many lines get drawn in that time.

Is this a Pencil problem? Is this a screen problem? My first run at the Genius Bar was inconclusive.

Does the community have any thoughts?