Hi Luis,
When combining a model attributes and value attributes in one context node it should be converted to a mixed node.
I found the solution in the meantime: the context node was bound to a context node in the custom controller. For some reason adding a value attribute to the context node in the view doesn't result in the conversion to a mixed node when this context node is bound. I removed the binding, then added the value attribute and then made the binding again and now the node is converted to a mixed node.
Kind regards,
John.