All discarded messages are stored in /var/lib/mailman/spam.
If a non-spam message accidentally gets discarded, just do:
-cat /var/lib/mailman/spam/not-really-spam.msg | /usr/lib/debbugs/receive
+/usr/lib/debbugs/receive < /var/lib/mailman/spam/not-really-spam.msg
chown Debian-debbugs:Debian-debbugs /var/lib/debbugs/spool/incoming/*
... check it works ...
mv /var/lib/mailman/spam/not-really-spam.msg /var/lib/mailman/not-spam/