Hi,
it seem that whether Kml, gpx or Geojson, the altitude of all trackpoints is set to 0, which is wrong on one side and on a second side, you don’t write it as null between <ele> and </ele> but you leave the field empty-
<trkpt lat=“48.082838000″ lon=“11.513063000”>
<ele></ele>
this generates an error by the .gpx file reading i.e in Basecamp garmin