mock called never gets set after upgrade to 2.1

[Julz]

after upgrading from 2.0.9 to 2.1.2 we have a problem with a test like this:

@patch.object(SomeModelAdmin, 'action_to_mock')
def test_admin_action_is_triggered(self, action_mock):
    url = reverse('admin:some_model_changelist')
    data = {'action': 'action_to_mock', '_selected_action': [1, 2]}
    self.client.post(url, data)
    self.assertTrue(action_mock.called)

on versions <2.1 it works fine. called is set to True and the call_count increases.
I can't find anything in the changelogs so I assume it's a bug