Case friendly, fits Spigen, Ringke, XOne and Poetic Cases

looks to be connector issue.
if you're lucky they just re-sit the connector and should be okey.
https://www.xda-developers.com/samsung-galaxy-s9-one-ui-2-1/

there's other updates in it.

If you still got warranty then better go claim now.
Did you phone got contact with water recently? Dont trust these phones water resistant especially after using for years. Recently my S8 screen also become weird after coming in contact with water. I didnt even dip it inside water, just some waterdrops splashed when watching youtube while bathing. It didnt show anything at first. Then I dismantle everything (noticed some water inside) and screen started showing up but not responsive to touches. After about 5 days or so, then the screen start showing display again but with some tint problem. Also certain area of the screen become deadzone and will not response to touches.

My wife S8+ did replace the screen before due to display issue as well. Just remembered it as well. IIRC, it all started with dim display like the display got a tint. Then will see lines like those you posted but her case still can see images. Since her set came with 2 years warranty, I just sent it in to replace screen FOC. I think it was about a year ago.

Anyway, AMOLED screen will slowly burn itself out. That is how it works. Having said that, my S6e which had a screen replacement few months into ownership still survived until today. I can see some screen burn-in though.

How much is S9+ still worth? Probably 800-1K? Would you dump RM700 to fix it? Even my S8 screen on Lazada cost over RM500. The only thing I liked about Samsung now is Dex. However with Google pushing on slowly with desktop mode, I guess I wont be missing Dex as much in a year or two time.

S9 Plus screen OEM LCD is around 670 and above. Labor add another 100.

Bitter pill reality.