After drawing using Graphics.SpriteBatch, why do my 3D objects draw incorrectly?
By default, SpriteBatch.Begin does not save your current render state, and will change certain render state properties that may make 3D objects render incorrectly. You can choose to either reset the render state yourself after the call to SpriteBatch.End, or call SpriteBatch.Begin and pass in SaveStateMode.SaveState, which will restore the render state after sprites are drawn.