Looks like this is the issue described in the comment here: https://github.com/python/cpython/blob/master/Lib/test/test_enum.py#L3691-L3692

On the first run you have the correct

  ('CONVERT_STRING_TEST_NAME_A', 5)

but later it turns into

  ('CONVERT_STRING_TEST_NAME_A', test.test_enum.CONVERT_STRING_TEST_NAME_A)

causing double-conversions of the enum elements. This causes the format(x) test to fail. You can re-create the same issue outside of the refleak by adding a simple:

    def test_convert_repr_and_str_again(self):
        self.test_convert_repr_and_str()

method.