Once again, bounce processing had stopped on phplist. My hunch is that this happens due to mails sent at times when bounce processing is supposed to take place - if the processing fails during the batch, then the next time the spool file is left in a way that processing fails again. So, as usual, the mitigation is to do the following as root:
rm /var/mail/the_relevant_spool_file touch /var/mail/the_relevant_spool_file
chown the_relevant_account:mail /var/mail/the_relevant_spool_file
chmod 660 /var/mail/the_relevant_spool_file
Once this is done, bounces are processed - if checking is necessary, can run the commandline process if a cron scheduled bounce process is at least half an hour away.