Thanks for the interesting information. I still have my old core module, so I’ll try downgrading to Android 9 Pie (LineageOS 16.0) and see whether I can get it working. I bought it factory-refurbished about 3.5 years ago and I always had some very mild dots on that core module but then the “screen splitting” started and after a few months it became unusable.
I have Android 10 (LineageOS 17.1 variety) installed on my “new” (second-hand) core module and it works without any issues, so that would mean there is a fault with the old core module that I haven’t got with the new core module.
I don’t know much about Android, but I’m wondering whether Android 10 uses parts of the hardware that Android 9 does not. There error message in the logcats I got would indicate that it might be graphics related. Would there then be a way to turn off whatever is different in Android 10 to get a working phone again?
Anyway, I’ll try Android 9 on the old module this week and report back.