you're overriding the barceloneta theme favicon but not the /favicon.ico which is stored in the registry.
This lines
don't use the Zope traverse to find the icon but lookup directly on the filesystem. So, no override. It works only for the viewlet (html), but you can see it when loading a file in the browser (pdf, an image) coming from Plone: the Plone icon is used.
This PR:
removed barceloneta-favicon.ico from the theme, so it can't be overridden as static resource
