I see what you mean now. In those cases where only the way was present (without existing multipolygon relation for the island), JOSM carried out all the copying.

In those cases you mentioned, the standalone admin_level=* tag left on the way was put into the relation role. As it was the practice in case of other islands (where only natural=coastline and source=* was left on the way). Some examples: https://www.openstreetmap.org/relation/17825226 ; https://www.openstreetmap.org/relation/4143917 ; https://www.openstreetmap.org/relation/3591870 ; https://www.openstreetmap.org/relation/6914328 ;https://www.openstreetmap.org/relation/1815263 ) 

Examples (best practice?) were checked, as I haven't seen exact statement on this (https://wiki.openstreetmap.org/wiki/Tag:place%3Disland ; https://wiki.openstreetmap.org/wiki/Tag:natural%3Dcoastline ; https://wiki.openstreetmap.org/wiki/Relation:multipolygon ; https://wiki.openstreetmap.org/wiki/Relation:boundary)

If there is a consolidated approach on this and they should be kept on the way instead of the multipolygon relation, I'm glad to put them back. 