Trouble with 30FPS Vsync while in fullscreen

I am running an old GTX 690 and am unable to achieve 60FPS at the settings I want to use. I decided that I would settle for 30FPS, but I am having trouble with the Vsync.

In my user.cfg file, I have the following:

sys_MaxFPS = 30
r_VSync = 1

When in windowed or borderless, I get vsync at 30FPS. As soon as I switch to fullscreen, vsync is still active, but the frame rate is no longer limited to 30FPS. I want to use full screen for Steam In Home Streaming (plus the extra FPS)

I’ve also tried using Nvidia Profile Inspector to force these settings, but the result is the same.Is this a bug or am I looking over something?