I think it's a bug on ActionController. I've created an issue on Inherited Resources203 for more details. And issue can be reproduce on Episode-196, which is a fork from Ryan Bates episode-196.
Basically, When STI and using nested form with f form object. the update method in controller didn't update the record and there's no error message.
This is actually a STI with nested attributes issue. I am not sure how to handle it is the best case. And I think it's more like a support question instead of bug here. So I would close it for now.