When I draw a polygon in QGIS, and export as a geopackage (or shapefile) in EPSG:6318, then try to import into Metashape while setting outerBoundary as the boundary type it imports successfully but does not set the outer boundary.
I can set the outer boundary after import manually or with a python script.
I'm pretty sure I tried it as a shapefile too and tried a manual import. In all cases I had to set the boundary after import.
sample gpkg and jpg attached.
jpg shows logfile with "boundary_type = OuterBoundary" and context menu with /Set Boundary Type/None checked (project state immediately after import)