Regarding conversion of Geojson format 3D buildings to cesium 3D tiles

We have multifloored building data in geojson format of different cities and we want them to be rendered at the front end as cesium tileset using cesium JS API.

I am facing issue with conversion of geojson data to cesium 3D tiles. I have tried converting geojson to cesium 3D tiles using FME software. When I convert using FME, some of the faces of buildings are missing (shown in the attached images). I have explored over web as well as cesium community, but I found cesium has built in tool for such conversion. Kindly guide me if something missing in that conversion or any specific tool required for such conversion.
I have attached images of geojson data as well as cesium 3d tileset data from all sides of the building.