Hi Alexander,
Please see, comparison in QGIS:
Please see opening output in mygeodata website failed.
Test data (input/output/QGIS)
5105 Kmz -TopoJson.zip (878.3 KB)
Thanks,...geometry can’t be render by viewer , but input kml is LineStringZ...see rusult in Qgis or other viewer you should use new option ‘TrimTo2DGeometry’...