We ended up finding the problem in our application that lead to the entities not displaying on the map after Cesium v1.108.0. It came down to Cesium not liking a country GeoJSON file that we were loading in the viewer. After we loaded that file using Cesium.GeoJsonDataSource.load then it caused polygons, polylines, corridors, CZML, etc. to not load fully on the map. We removed the loading of that file and everything now works as expected.
We will be looking into finding a new country GeoJSON file to use, but it seems like lots of other users experience problems with country files as well: Overlay country data with GeoJSON