There are two issues here. To me, the most important one is the past water damage. You say that someone cleaned it but do you know what they did to clean it? A proper decontamination requires that the shields be removed from the logic board, that it be cleaned in a lab-grade ultrasonic cleaner with special chemicals. Some people just take IPA and a toothbrush and call it a day; these phones tend to suffer from latent issues downstream as the oxidation continues underneath the IC's over the following weeks and months.
However, even with the best process, most iPhones still require some micro-soldering to replace defective or heavily oxidized components. Sadly, many phones simply cannot be recovered economically.
All this to say that your current problem could simply be a defective screen but given the history of the phone, it may be a logic board issue. The only way to know is to replace the screen to see if it solves your problem. IMO, I don't think its worth wasting more money on a previously water damaged phone.