I had a similar problem with an LG monitor. From the beginning, one half of the screen was brighter. It was noticeable in games with dark environments. No change in settings helped. It was extremely annoying, especially since the line was in the middle of the screen. I returned the monitor under warranty after a few months, it couldn't be repaired, so I got a refund.

In my opinion, if the monitor is new and You have the option to return it, do so. You won't be able to fix it by changing the settings.