@ All : Black Lines = Vanilla Function.

They're present in the Vanilla Title Screen Background as well. But they're made in a way that you can't see them unless you have your face against the monitor. The lines are the division sections between the multiple images used to create the background. Of which there are 8 or 9. Each is a section of the screen both With & Without Lightning Flash effect.

The screen is divided into 4 sections. A vertical line is seen in the middle. But a harder to notice horizontal line also exists fairly close to the bottom (20% ish above the bottom). That creates the 4 sections that can flash (be replaced by brighter versions of the image) during the Thunder+Lightning effect of the main menu music & background.

But worry not- the lines are normal. You can verify this by trying any other main menu background replacer and you'll see the same lines. I believe its tied to Resolution UpScaling as it's much more noticeable in 2K and 4K, etc.

Hope this helps ease some concerns and confusion. Unfortunately, not much can be done except running PZ in 1080p FullScreen. >.<