Query for boundary relations with missing "admin_centre" roles

gistfile1.txt

/*
Question: Boundary relations without "admin_centre" role nodes
Overpass Turbo query: http://overpass-turbo.eu/s/17ef

For admin_level details, see https://osm.org/wiki/Philippines/Mapping_conventions#Administrative_boundaries
*/

[out:json][timeout:600];

area[admin_level=2]["name"="Philippines"][boundary=administrative]->.NominatimArea;

( rel(area.NominatimArea)[boundary=administrative][admin_level=6]; - 
 
  ( node(r:"admin_centre")->.a;
    rel(bn.a)[boundary=administrative][admin_level=6]; 
  );
);

out geom;