Good catch! Thank you Raymond for the third example.

I though that it can be easily fixed by calling tp_traverse() for submapping:

 static int
 mappingproxy_traverse(PyObject *self, visitproc visit, void *arg)
 {
     mappingproxyobject *pp = (mappingproxyobject *)self;
-    Py_VISIT(pp->mapping);
-    return 0;
+    return Py_TYPE(pp->mapping)->tp_traverse(pp->mapping, visit, arg);
 }

but it does not work.

So I am okay with closing this issue.