This is fixed in Corona 13 Hotfix 1 - https://forum.corona-renderer.com/ Chaos Corona 13 (Hotfix 1) now available for 3ds Max and Cinema 4D
As Tom mentioned, diffuse + roughness should be working fine now, but there are some combinations which may not be working correctly yet (e.g. roughness + IOR or roughness + metalness). We are working on fixing this as well.