When -titleregex is called with a namespace other than 0 (with GeneratorFactory), no pages are returned. Only pages with namespace 0 are ever returned.
In GeneratorFactory, -titleregex calls RegexFilterPageGenerator (wich is really RegexFilter.titlefilter) with an argument of site.allpages(). allpages defaults to namespace 0. When getCombinedGenerator() is called, the generator (already filled with namespaces of 0) is not considered a pywikibot.data.api.QueryGenerator and is filtered out with NamespaceFilterPageGenerator for the appropriate namespaces, either resulting empty results or only with namespaces 0.