Go to System_Preference>Accessability>Display and check Reduce transparency. This made them visible for me. I then went back and unchecked. The issue recurred later and I want through the exercise again. So there is a bug there, and I have reported via Feedback Assistant. Hope this is helpful for you.
bobscape
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I had the same issue. It was resolved when I turned off (unchecked) Reduce Transparency. (System Preferences > Accessibility > Display > Reduce Transparency )