Hi Xaric!
The ribbon cables look fine to me tho. The upper LCD screen ribbon cable is somewhat dirty and you could try to clean it and see if that works. Still, you would expect the upper screen to fail if the ribbon cable is damaged. Are you sure that the bottom replacement screen was OK? Maybe a faulty replacement? Could be a thing.
I would replace that one again for a new one and see how that goes. As you said, it was first glitchy and then failed.. does really sound like just the bottom screen to me.
If it failed again, after trying a new bottom screen again, then it’s most likely on motherboard level. But nothing is for sure, you can try to replace the upper screen, but I don’t think this will solve issues with the bottom screen. But you can only be sure if you test other components out.
Please let us know how it goes!