Have to tell Squid to be sure to check for new package lists, so that it does not serve old packages.
To do that change the refresh pattern option for “refresh-ims”.
refresh_pattern Packages\.bz2$ 0 20% 4320 refresh-ims refresh_pattern Sources\.bz2$ 0 20% 4320 refresh-ims refresh_pattern Release\.gpg$ 0 20% 4320 refresh-ims refresh_pattern Release$ 0 20% 4320 refresh-ims