Me too, but it wasn't globally true. I ran:
$ pdsh -lroot -g os=debian_linux '[ "$(lsb_release -cs)" = "jessie" ] && { systemctl -q is-active cron.service || { echo CRON_NOT_RUNNING; systemctl restart cron ;} ;}' | tee /tmp/jessie-cron.log
(i use PDSH to remote manage several hundred Debian systems, most still Wheezy.)
and found a good chunk of my Jessie desktops with unattended-upgrades were hit by this.
(but not all). Also, my virtualbox Jessie instance here at home was upgraded by apper via notifications interactively (not unattended-upgrades) and 'cron' restarted fine there. (perhaps the 'cron' package has already been fixed?)
$ systemctl status cron
● cron.service - Regular background program processing daemon
Loaded: loaded (/lib/systemd/system/cron.service; enabled)
Active: active (running) since Sun 2015-09-06 10:56:34 MDT; 36min ago
Docs: man:cron(8)
Main PID: 12431 (cron)
CGroup: /system.slice/cron.service
└─12431 /usr/sbin/cron -f
--stephen