I have the same issue since the update on my S20+. I also turned on the 'Show refresh rate' in developer mode to see what's happening and stays flat at 60 even if I choose the 120 Hz refresh rate in the display settings
I have managed to fix it! You need to change the resolution to 3200 x 1440 then change back to 2400 x 1080. Then change refresh rate back to 120hz and it should work!