Happen on MacOS 10.14.6 (18G84).
When using Spotify on full screen, the top desktop thumbnail is missing and we get just a black square.
It used to work OK a few months back.
Tried to reinstall spotify, I updated MacOS several times on the last few months.
After sawing this phenomenon on other computers of my colleagues I decided to share this issue.
It feels like the app is not rendered while not on focus.
Once going back to Spotify the UI renders and you can see the black background for few milliseconds before it loads.
* Easy to reproduce.
Attached image showing the black screen.