Hello felidaelynx,
As you can see here:
https://tools.geofabrik.de/osmi/?view=areas&lon=6.78823&lat=46.22018&zoom=12&baselayer=Geofabrik%20Standard&opacity=0.14&overlays=duplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways%2Cduplicate_node%2Csingle_node_in_way%2Cduplicate_segment%2Cway_in_multiple_rings%2Cintersection%2Cintersecting_segments%2Cring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways
The use of a multipolygon is not appropriate for defining the area as it generates several alerts.
The sub-areas do not overlap but are adjacent, and since they have different names, they cannot be combined into a single area with a single name, which is a characteristic of a multipolygon.
I would like to point out that the relation had already been correctly entered as a type:site relation, so I do not understand why you want to transform it into a type:multipolygon relation, which is not appropriate in this case.
Best regards