If the machine was suddenly shutted down (hard reboot for example) while
processing core dump, temp files created manually (not with a O_TEMPFILE flag)
stay in the system. After reboot systemd-coredump treat them as usual files, so
they wouldn't be rotated and shall pollute the filesystem.
Solution is to simply add those temp files to systemd-tmpfiles configs.
# Remove top-level private temporary directories on each boot
R! /tmp/systemd-private-*
R! /var/tmp/systemd-private-*
+
+# Handle lost systemd-coredump temp files. They could be lost on old filesystems,
+# for example, after hard reboot.
+x /var/lib/systemd/coredump/.#core*.%b*
+r! /var/lib/systemd/coredump/.#*