Yeah, that's what the default in "Hide User Agent" in case it needs to restore the original. I never changed that, just hide or show the agent. And I never had this problem before the update to PR 1.3.
Mozilla/5.0 (X11; U; Linux armv7l; en-GB; rv:1.9.2b6pre) Gecko/20100318 Firefox/3.5 Maemo Browser 1.7.4.7 RX-51 N900