Flex cables is certainly a problem as well (not all models have the main and sub connected via flex cable as you certainly know).
I remember Samsung sending us a batch of A-series phones where the flex cables were bad and needed to be swapped before they even got sold. (Official repair center). We also had batches of these faulty flex cables that where causing green lines when repairing the a-series like the A15.
I also worked on display repair for the S-series, S7 -> S20, where a new screen and connector is placed on the old bracket, and it can happen there too when the screen or connector, has minimal damage. Especially when you get refurbed parts from Samsung. These come in dreaded blue boxes instead of white and where prone to have lines and other faults in them. Fucks up the targets…