Ubuntu 20.04's mdadm package doesn't appear to have a crontab entry for scrubbing actually (while 18.04 does). It does have e2scrub_all, but does that serve the same purpose?
mdadm scrubbing is in various states of broken across major distros. It used to be that each distro had their own cron-based scrubbing scripts. At some point mdadm introduced its own checks using systemd timers and there have been bugs upstream and in packaging.
I only noticed because I'm preparing to upgrade an old home server and carefully double checking everything. I'll probably end up grabbing one of the old cron scripts and setting it up manually. Very disappointing.
Debian/Ubuntu: https://bugs.launchpad.net/ubuntu/+source/mdadm/+bug/1852747
RHEL/Centos: https://bugzilla.redhat.com/show_bug.cgi?id=1774354