We believe that the bug you reported is fixed in the latest version of gnubiff, which is due to be installed in the Debian FTP archive:
gnubiff_2.1.1-3.diff.gz to pool/main/g/gnubiff/gnubiff_2.1.1-3.diff.gz
Well I tested the new package and I sorry to say that the bug is still there. The configuration file has not changed and the behavior of the bug is the same:
gnubiff checks the mailbox when it starts and then it does not check them again.
-- (°> Nicolas Évrard / ) Liège - Belgique ^^