I'm about to pull my hairs on this. Trying to use the material override on Cycles as I did on Blender version 2.79. Many files work fine, but there are two that are giving me a lot of issues and I can't find a solution.
When I apply the the material override I can see it while previewing the render in the viewport (Viewport Shading: Render):
But when I send to render the material override property is completely ignore it.
I have saved the files as copies since I updated to 2.80 recently making sure I don't loose the originals. So the problem just shows on 2.80 but not with every files.
If I send to render with Render Single Layer enable , then shows the material override but ignore Freestyle.