just an update after re-installing Power BI Desktop (download-able version):

if you check under C:\Program Files\Microsoft Power BI Desktop\bin\ODBC Drivers\Simba Spark ODBC Driver

you will see that it is actually a more recent version (2.6.18.1030) which should support cloud fetch

I had an older version of Simba Spark driver installed manually before - dont know which version Power BI was using then - but I uninstalled this one now and now Power BI can only use the most recent one it comes with