many thanks for your report. It seem cron is getting killed by the system. Can you have a look to journalctl and kernel messages if you see some errors related to killing cron?
journalctl -u cron.service
-- Logs begin at Thu 2016-11-03 18:16:42 CET, end at Tue 2020-05-19 17:57:14 CEST. --
May 19 17:52:12 DietPi systemd[1]: Started Regular background program processing daemon.
May 19 17:52:12 DietPi cron[1114]: (CRON) INFO (pidfile fd = 3)
May 19 17:52:12 DietPi cron[1114]: (CRON) INFO (Running @reboot jobs)
May 19 17:52:12 DietPi CRON[1121]: pam_unix(cron:session): session opened for user root by (uid=0)
May 19 17:52:12 DietPi CRON[1120]: pam_unix(cron:session): session opened for user root by (uid=0)
May 19 17:52:12 DietPi CRON[1119]: pam_unix(cron:session): session opened for user root by (uid=0)
May 19 17:52:12 DietPi CRON[1136]: (root) CMD (/root/.configrc/b/sync>/dev/null 2>&1 )
May 19 17:52:12 DietPi CRON[1137]: (root) CMD (/usr/sbin/logrotate /etc/pihole/logrotate)
May 19 17:52:12 DietPi CRON[1122]: pam_unix(cron:session): session opened for user root by (uid=0)
May 19 17:52:12 DietPi CRON[1141]: (root) CMD ( PATH="$PATH:/usr/local/bin/" pihole updatechecker remote reboot)
May 19 17:52:12 DietPi CRON[1120]: (CRON) info (No MTA installed, discarding output)
May 19 17:52:12 DietPi CRON[1145]: (root) CMD (/root/.configrc/a/upd>/dev/null 2>&1)
May 19 17:52:12 DietPi CRON[1120]: pam_unix(cron:session): session closed for user root
May 19 17:52:12 DietPi CRON[1122]: pam_unix(cron:session): session closed for user root
May 19 17:52:12 DietPi systemd[1]: cron.service: Main process exited, code=killed, status=9/KILL
May 19 17:52:12 DietPi systemd[1]: cron.service: Unit entered failed state.
May 19 17:52:12 DietPi systemd[1]: cron.service: Failed with result 'signal'.
root@DietPi:~# journalctl -u cron.service
-- Logs begin at Thu 2019-02-14 11:11:58 CET, end at Sat 2020-08-29 11:42:01 CEST. --
Aug 29 11:40:41 DietPi systemd[1]: Started Regular background program processing daemon.
Aug 29 11:40:41 DietPi cron[824]: (CRON) INFO (pidfile fd = 3)
Aug 29 11:40:41 DietPi cron[824]: (CRON) INFO (Running @reboot jobs)
Aug 29 11:41:01 DietPi CRON[1118]: pam_unix(cron:session): session opened for user root by (uid=0)
Aug 29 11:41:01 DietPi CRON[1119]: pam_unix(cron:session): session opened for user root by (uid=0)
Aug 29 11:41:01 DietPi CRON[1126]: (root) CMD (cd / && run-parts --report /etc/cron.minutely)
Aug 29 11:41:01 DietPi CRON[1127]: (root) CMD (/cmd/kikapcs.sh)
Aug 29 11:41:01 DietPi CRON[1118]: pam_unix(cron:session): session closed for user root
Aug 29 11:41:01 DietPi CRON[1119]: (CRON) info (No MTA installed, discarding output)
Aug 29 11:41:01 DietPi CRON[1119]: pam_unix(cron:session): session closed for user root
Aug 29 11:42:01 DietPi CRON[1258]: pam_unix(cron:session): session opened for user root by (uid=0)
Aug 29 11:42:01 DietPi CRON[1259]: pam_unix(cron:session): session opened for user root by (uid=0)
Aug 29 11:42:01 DietPi CRON[1266]: (root) CMD (/cmd/kikapcs.sh)
Aug 29 11:42:01 DietPi CRON[1267]: (root) CMD (cd / && run-parts --report /etc/cron.minutely)
Aug 29 11:42:01 DietPi CRON[1259]: (CRON) info (No MTA installed, discarding output)
Aug 29 11:42:01 DietPi CRON[1258]: pam_unix(cron:session): session closed for user root
Aug 29 11:42:01 DietPi CRON[1259]: pam_unix(cron:session): session closed for user root