Perhaps you want to explain the steps to reproduce a little better. It shouldn't be an issue to create a new feature and tick the multiple values box for it. As soon as e.g. an attribute for an item gets derived from it within the group you are able to set multiple values for it.
If you want to make a field multi-valued, upper bound property should be set to "-1" for that specific field in the repository. and try the same for attributes also and see if that helps?