Its not a bug, if your on fullscreen (or borderless fullscreen) your output render target needs to be the native resolution of your monitor. Currently it’s drawing your output to the position on screen of your monitors native resolution.
If you want your output resolution to be smaller than your monitors native resolution you need to output the scaled resolution your self.
You can query the current full screen display resolution by switching to full screen and gettimg GraphicsAdaptor.DefaultAdapter.CurrentDisplayMode.Width (or height)
In my games in my video settings when changing to fullscreen I revert the resolution to that full screen resolution.