Monitor stuck at 120Hz when Fullscreen native

For anyone running into this thread in the future, this will fix your issue: https://www.reddit.com/r/Monitors/comments/1042u1s/lg_oled_27gr95qebaus_says_resolution_is_incorrect/j33kerg/

"I had luck by using CRU 1.5.2 (https://www.monitortests.com/forum/Thread-Custom-Resolution-Utility-CRU). When you launch it, select the dropdown for your LG monitor and export the settings to have a safe copy (export button in bottom left). Click on the Extension Block "CTA-861" and a new pop up should appear titled "Extension Block." in the bottom of the two windows double click "TV Resolutions" and in the window that appears select "Delete All" on the bottom line. After this, close CRU, and run "restart64" which is included in the CRU download folder. This resets your graphics driver, and your monitor(s) will briefly black screen (this is fine). Once the driver is restarted, if you go into NVidia Control Panel you should see the Ultra HD, HD, and SD resolutions sections should be gone, and the max offered resolution should be 2560x1440. This should address the issue of full screen applications changing the resolution and dropping the frame rate. Time will tell if this is permanent or if it will need to be repeated.

Very unhappy that I had to do this to get the monitor to function properly with my system."

This also goes for Valorant and other games with the same fullscreen native issue.