@ObsoleteSony I just received the digital version of the book. After a briefly go through, I must say that I am pleased and impressed by the work you have done. Congratulations on the release!
This issue affects all custom wallpapers on iOS and iPadOS 26.
When an image is set as wallpaper, it is cropped to match the display’s exact resolution. So when your screen is locked, black bars appear on all four sides due to the parallax effect in the system.