I recently started updating to MonoGame 3.6 (boo technical debt!) but I've noticed that none of my "games" that use rendertargets are rendering correctly... It seems that between MonoGame 3.5.1 -> 3.6 the RenderTargetUsage parameter is being ignored.
This is a DesktopGL tool that worked perfectly in 3.5.1, but after updating to 3.6 clears the screen contents to black. I specifically have the following code in the app:
//When using render targets, don't clear the screen!!!
GraphicsDevice.PresentationParameters.RenderTargetUsage = RenderTargetUsage.PreserveContents;
which is being ignored.