Fixed #27993 -- Fixed model form default fallback for SelectMultiple.

django/forms/widgets.py

@@ -714,6 +714,11 @@ class SelectMultiple(Select):
             getter = data.get
         return getter(name)
 
+    def value_omitted_from_data(self, data, files, name):
+        # An unselected <select multiple> doesn't appear in POST data, so it's
+        # never known if the value is actually omitted.
+        return False
+
 
 class RadioSelect(ChoiceWidget):
     input_type = 'radio'