This is a Windows 8 bug, not a MC bug, and all we can do is try to workaround the issue, we cannot properly fix it.
As such, the workaround we use (and madVR uses) works fine for a majority of people, for me on both NVIDIA and Intel GPUs, don't have a Win8 system with AMD to test.
If the workaround doesn't work for you, and you are certain that you're not seeing something unrelated, I suggest you complain to Microsoft.
Sorry, but thats all we can do. They broke the refresh rate API - we ask for 24 Hz, and Windows switches to 23.
Also, fullscreen mode in MC isn't any special really, its just a maximized window, there is no resolution change (unless you use madVR with Fullscreen Exclusive Mode), so I'm not sure there can be any difference between windowed and fullscreen.
As such, I'm not convinced that you are really referring to refresh rates here. How do you detect the difference between 23.976 and 24.000 in RO Standard mode, without the madVR OSD to tell you?
Edit:
I just double checked in ROHQ, because the madVR OSD immediately show the info.
Starting a video from Theater View directly in fullscreens gives proper 24.000 for me.