No Update of Fields after Validation error
This problem is related to JSF issue 1060 and in detail elaborated in this Q&A: How can I populate a text field using PrimeFaces AJAX after validation errors occur?
To the point, this behaviour is (unfortunately) correct as per current JSF specification and this can be solved as follows: during the invoke action phase you need to collect UIInput components which are included in the ajax render, but not in the ajax execute, and then invoke resetValue() method on them. This can be in a generic and reuseable fashion be done with help of OmniFaces ResetInputAjaxActionListener.