So, you are sending a broadcast via Buddies > New Instant Message menu in Pidgin? Then you enter groupname@broadcast.servername and then put the message and hit Enter?
You are logged in Pidgin with the user which is added to Openfire administrators via System Properties or by directly editing openfire.xml or maybe with the built-in admin user?
I have tested those various scenarios with Pidgin sending broadcasts and logged in in another Pidgin with an administrative user, which is not a member of a group a broadcast is been sent to, and i still can't reproduce your issue.