Thanks!
Unfortunately, this is a different issue than the one identified in Cesium v1.110 Introduced Problems with Polygons Containing Holes - #4 by Sequoiaandrade. That example specified holes with the PolygonHierarchy option holes, which I don’t see being used here.
Instead, this approach relies on a sort of artifacts of polygon rendering where overlapping to show the holes. Technically though, these polygons are not simple, and therefore not officially supported by CesiumJS since the triagulation library we use under the hood, earcut, does not support these kinds of polygons.
I think it was a happy accident that these were rendering correctly in previous versions. Going forward, I would reccomend simplifying your geojsons before visualizing them in CesiumJS. Turf.js’s unkinkPolygon is one such tool you could use.