1. A concise explanation of the problem you're experiencing.
Cesium 1.50 failing to display imagery layers when using a non-earth ellipsoid.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
While the minimal example above of displaying the lunar WMS datasource doesn't specifically require the definition of a custom ellipsoid, I maintain a lunar map with many features that do depend on it. I recently attempted to upgrade and found all the imagery layers not showing while vector features seemed to continue to display properly.
4. The Cesium version you're using, your operating system and browser.
Cesium 1.50, Ubuntu 16.04, Chrome 69.0
Thanks in advance for any help on this matter!