src/java.base/share/classes/sun/util/locale/provider/FallbackLocaleProviderAdapter.java

@@ -64,6 +64,11 @@
 
     @Override
     protected Set<String> createLanguageTagSet(String category) {
         return rootTagSet;
     }
+
+    @Override
+    public boolean isSupportedProviderLocale(Locale locale, Set<String>langtags) {
+        return Locale.ROOT.equals(locale);
+    }
 }