You should ask this question at the monogame forums, or file a bug against them with a simple repro project.
I've definitely encountered a few issues with graphics device setup in monogame, but was able to work around them (and filed bugs so they got fixed). It's possible the current version of monogame fixes whatever issue you're running into (assuming you're running the latest official release). You can also build monogame yourself... I had to do this for an audio issue whose fix hadn't been pushed into the official source code yet.
fyi, I just tested IsFullScreen with a Monogame (v3.0) Windows OpenGL project, and it worked fine. I also don't see any issues with GraphicsAdapter.CurrentDisplayMode.