I posted a bug in github but I thought I’d bring it up here in case anyone else ran into this, or in case there was a known fix. as of MonoGame 3.7 (early August 2017), this seems to not be fixed.
The summary is that the interaction of the GraphicsDevice.Viewport and GraphicsDevice.ScissorRectangle does not render correctly in MonoGame, but it does in XNA. Full description, sample code, and screenshots can be found in the github issue:
Has anyone encountered this or does anyone know of a fix?