1. Enable forward shading by check the Forward Shading box under Edit -> Project Settings -> Engine - Rendering -> Forward Renderer.
2. Change "Anti-Aliasing Method" to "Multisample Anti-Aliasing (MSAA)" under Edit -> Project Settings -> Engine - Rendering -> DEfault Settings)

You'll see serious dithering artifacts in all Cesium objects, while objects that user other materials look fine:

Deleting the DitherFade layer from our default material fixes the problem. We're using an engine-provided material node that is meant for temporal AA, but it's not clear why it fails so badly with MSAA.

Originally reported on the forum:
https://community.cesium.com/t/forward-rendering-msaa-black-squares-and-dithering-artifacts-on-terrain/24149