Same problem to me. It seems that after Windows 10 finishes with the last updates, 300 Hz setting disappears. I've restored my backup imagine saved two months ago and everything goes back to normal. I did this for 3 times to find out what makes 300hz setting to disappear and it is happening again and again after windows 10 installs some driver updates, including latest NVIDIA driver which doesn't seem to be his fault. Let me know when you find something useful. Regards!
AMD GPU latest driver brings this problem. Just uninstall it and reinstall the AMD_Graphic_DriverOnly_ROG_AMD_Z_V27.20.14016.5003_21901 version.