When I looked at first, the bounce runner process had died and not been restarted. Then I restarted mailman3 and now the bounce runner is using a ton of CPU but the queue is still going up.
root@lists1001:/var/lib/mailman3/queue/bounces# ps -axjf | grep runner 14967 14969 14967 14967 ? -1 S 38 0:01 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=archive:0:1 14967 14970 14967 14967 ? -1 R 38 18:11 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=bounces:0:1 14967 14971 14967 14967 ? -1 S 38 0:01 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=command:0:1 14967 14972 14967 14967 ? -1 S 38 0:03 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=in:0:1 14967 14973 14967 14967 ? -1 Sl 38 0:09 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=lmtp:0:1 14967 14974 14967 14967 ? -1 S 38 0:01 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=nntp:0:1 14967 14975 14967 14967 ? -1 S 38 0:07 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=out:0:1 14967 14976 14967 14967 ? -1 S 38 0:02 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=pipeline:0:1 14967 14977 14967 14967 ? -1 S 38 0:01 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1 14977 14997 14967 14967 ? -1 R 38 0:22 | \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1 14977 14998 14967 14967 ? -1 S 38 0:21 | \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1 14967 14978 14967 14967 ? -1 S 38 0:00 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=retry:0:1 14967 14979 14967 14967 ? -1 S 38 0:01 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=virgin:0:1 14967 14980 14967 14967 ? -1 S 38 0:02 \_ /usr/bin/python3 /usr/lib/mailman3/bin/runner -C /etc/mailman3/mailman.cfg --runner=digest:0:1
I tried starting the runner manually with a bit more output: