proper multipolygon member roles completed

Why are you splitting simple polygons into individual snippets just to then pack them into a multipolygon relation? This creates additional, unnecessarily complex objects. In order to treat the snippets as a area, they have to be reconstructed into polygons anyway.

Please check and revisse your edits:
https://tools.geofabrik.de/osmi/?view=areas&lon=15.01951&lat=49.22972&zoom=11&baselayer=Geofabrik%20Standard&opacity=0.95&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

Da ist wohl einiges schiefgegangen:
https://tools.geofabrik.de/osmi/?view=areas&lon=9.78917&lat=52.39608&zoom=14&baselayer=Geofabrik%20Standard&opacity=0.95&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

I get at least a ‘Split way confirmation’ dialogue box.

If not all parent relations are loaded, JOSM does not know about the relation membership of the split way.

In order to determine whether the members of a boundary relation form a closed polygon, they must be completedly loaded.

https://tools.geofabrik.de/osmi/?view=areas&lon=6.92583&lat=50.58291&zoom=11&baselayer=Geofabrik%20Standard&opacity=0.95&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

Bitte bei der Bearbeitung von Relationsmitgliedern beachten, dass diese auch Mitglirder von anderen Relationen sein können. Diesbzügliche JOSM-Warnmeldungen nicht ignorieren. Vor dem Hochladen JOSM-Validator benutzen und Fehler korrigieren!

Please check https://tools.geofabrik.de/osmi/?view=areas&lon=-116.30614&lat=49.10096&zoom=12&baselayer=Geofabrik%20Standard&opacity=0.95&overlays=duplicate_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 and fix issues before further editing.

Please check the outcome of your edits and fix iit if necessary: https://tools.geofabrik.de/osmi/?view=areas&lon=-74.09536&lat=46.05410&zoom=12&baselayer=Geofabrik%20Standard&opacity=0.95&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

Das geht auf das Konto von ID, das offene ways, getagged als boundary=postal_code zu Polygonen schließt. Siehe https://community.openstreetmap.org/t/ungefragte-autokorrektur-durch-id/132903

There is no need to delete objects tagged as abandoned

Siehe https://tools.geofabrik.de/osmi/?view=areas&lon=12.49961&lat=50.71759&zoom=15&baselayer=Geofabrik%20Standard&opacity=0.95&overlays=ring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways
Grenzrelationen verstümmelte durch Löschung von way/571464759

Please check and fix. JOSM/Validator might be helpful.
https://tools.geofabrik.de/osmi/?view=areas&lon=-74.20714&lat=45.68955&zoom=12&baselayer=Geofabrik%20Standard&opacity=0.95&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

Ist dir bewusst, was Du mit deinen Änderungen bewirkst? Hast Du ein Konzept, wie Du die Informationen, die in den verstümmelten Relationen stecken, wieder herstellt?
https://tools.geofabrik.de/osmi/?view=areas&lon=9.05776&lat=51.27667&zoom=15&baselayer=Geofabrik%20Standard&opacity=0.95&overlays=ring_not_closed%2Ctouching_rings%2Crole_should_be_inner%2Crole_should_be_outer%2Cinner_with_same_tags%2Cways

Löschung des ways way/279108125 hat die Relation relation/3710974/history kaputt gemacht.