Was there a solution to this?
To all the future people that despair to find a solution to this BUG that has not yet been fixed (5 years in bug tracker), I will provide you with an actual workaround compatile with Excel (not solved by me).
It is in acomment in that link : https://enterprisesmartapps.wordpress.com/2011/12/26/birt-drop-group-header-property-with-doc-output-type/
I join a report with the workaround.
Worth to be noted, it only works if you have one grouping to do.