I only quickly looked over this for now (and will have to allocate some time to inspect the data more thoroughly, and try out the conversion).
But the last sentence…
sounds oddly familiar: It reminded me of EXT_mesh_features extension ignoring my class/table definitions, always looking up in propertyTable at index 0 · Issue #11683 · CesiumGS/cesium · GitHub .
(Maybe you can say from the tip of your head whether that would fully explain the observed behavior. Otherwise, it may be necessary to investigate this further)