This can either be a bad LCD, or a bad LVDS cable going from the display to the driver board. In some cases, the controller board will be at fault.
First thing to check is the cable - source out a cable, swap it out and see if the issue gets resolved. If it does not, then you’re sadly looking at an expensive repair :-(. Either you’ll need to replace the LCD ($$$ due to LG panel cost), or the driver board ($$ so not as bad but still expensive. As far as parts go, check sites like eBay as well as scrappers who resell the parts from these.
The issue with repairing these beyond a new LVDS cable is the parts cost will more or less rival a used replacement, or a Thunderbolt upgrade on capable Macs.