See
https://github.com/koreader/koreader/issues/5732. It is a non-fixable thing.
You can, of course, hack the code to write the screensaver to a jpg/png file each time a new document is opened. I would suggest to develop that on the emulator, not directly on a android device.
When you manage to modify the code you'll need to import your modified files in the program each time the program is updated. You can use a modified version of
https://github.com/koreader/koreader...ation-patterns