There were processes running inside the chroot at the end of the piuparts run.
This is probably due to directly calling /etc/rc.d/ scripts in packages maintainer scripts, which is a violation of policy 18.104.22.168 and must be replaced by using invoke-rc.d (which will respect an optionally existing policy-rc.d) - see
https://www.debian.org/doc/debian-policy/ch-opersys.html#interfacing-with-init-systems, /usr/share/doc/sysv-rc/README.invoke-rc.d.gz and /usr/share/doc/sysv-rc/README.policy-rc.d.gz.
According to the thread started at firstname.lastname@example.org these bugs are to be filed with severity serious.
The commandline to find these logs is:
COMMAND='grep -E "ERROR: FAIL: Processes are running inside chroot"'