Along with the above information, we have tried using bindings for both alias-binding and parameter-binding in the *-object.xml file and the outcome was always the same.
Public Render URLs are a special type of Render URL as defined by the JSR286 (Portlet 2) specification. They are retained through multiple renders (and re-renders) of a given portlet and other portlets setup to use the shared parameter.
This is standard behavior for render parameters in the portal environment. See the JSR286 spec for more info: http://jcp.org/en/jsr/detail?id=286