Finally able to reproduce the problem of ghost dialogs that stick around after being closed, with the add a custom new source dialog. Workaround of calling deleteLater() seems to do the trick.

src/calibre/gui2/dialogs/scheduler.py

@@ -491,8 +491,11 @@ class Scheduler(QObject):
     def customize_feeds(self, *args):
         from calibre.gui2.dialogs.user_profiles import UserProfiles
         d = UserProfiles(self._parent, self.recipe_model)
-        d.exec_()
-        d.break_cycles()
+        try:
+            d.exec_()
+            d.break_cycles()
+        finally:
+            d.deleteLater()
 
     def do_download(self, urn):
         self.lock.lock()

src/calibre/gui2/metadata/single.py

@@ -1048,7 +1048,8 @@ def edit_metadata(db, row_list, current_row, parent=None, view_slot=None,
                 set_current_callback=set_current_callback)
         return d.changed, d.rows_to_refresh
     finally:
-        d.hide()  # possible workaround for bug reports of occasional ghost edit metadata dialog on windows
+        # possible workaround for bug reports of occasional ghost edit metadata dialog on windows
+        d.deleteLater()
 
 if __name__ == '__main__':
     from calibre.gui2 import Application as QApplication