If you add another PPIO to the WPS that has a name that starts with a letter before W it will be picked for WFS sources that only declare thier mime-type as 'text-xml' as none of them match that. Note this is what the request builder uses as the mime type for local layers.
Issue occurs in org.geoserver.wps.ppio.ProcessParameterIO.find(Parameter<?>, ApplicationContext, String)
Just picks the first alphabetically.