Not sure what's different but I tested your dataset (it's just 14 countries boundaries with their iso codes) and it imported fine. I used Kibana 8.3.3 on the Elastic Cloud.
Maybe you want to run your geojson file trough mapshaper to clean up any geometry issues. This command works well:
mapshaper -i fin.geojson -snap precision=0.000001 -clean -simplify percentage=99% -o fin_clean.geojson
(you can also use their website graphical interface for this)