By “latest Corona” do you mean 7 Hotfix 1, or 8 Release Candidate 4? People can interpret that different ways (or sometimes not be aware that there is another version out so think that RC2 is the latest), always best to state the exact version please.
Those are not Corona previews in the material editor. Are you 100% sure that you have Corona set as your active renderer?
If so, can you scroll down in Render Setup > Common and share a screenshot of how the “Assign renderer” rollout looks like for you?