Its been a while sinse I’ve used plasma, I had the same issue but I was using nvidia at the time and nvidia has a tool to set refresh rate, screen size and a lot of other things, gsync/freesync being one of them. Now most monitors do have if gsync/freesync enabled an option to turn it off. but before you do that if you have other display port/ HDMI cables around then switching though them would be advantageous as youll avoid screen tearing