Thanks! I can reproduce this, and I have a workaround.
Open up Materials/Instances/MI_CesiumThreeOverlaysAndClipping with the UE Editor. It’s inside the Cesium for Unreal plugin folder, so you might have to enable “Show Plugin Content” in the Content Browser.
Then find the DitherFade material layer and delete it by pressing the X:
Save the material, and the dithering artifact should go away.
The problem is caused by the use of the engine-provided DitherTemporalAA node. As the name implies, it’s meant to be used with temporal anti-aliasing. But why it causes that dithering artifact with MSAA - when the dithering should be inactive anyway! - is a mystery to me. I’ll investigate further.