Ho provato con lo strumento "Rendi valide le geometrie" di OpenJUMP e restituisce la geometria corretta come multipoligono. OpenJUMP sta usando la propria funzione makeValid. Non so se QGIS abbia una funzione equivalente ma poiché hai già la funzione in PostGIS puoi semplicemente usare ST_MakeValid http://postgis.net/docs/ST_MakeValid.html .
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