Comment # 1 on bug 920205 from Neil Brown

The script does not report mismatches.
Detecting mismatches isn't really the point of running a 'check'.
The main point is to read all block and make sure that none of them have gone
bad (i.e. cannot be read).
If any blocks are bad, they will automatically be fix (re-writing) if possible.
 If that isn't possible, the drive will be removed from the array.

Mismatches *are* reported by "mdadm --monitor".
When "mdadm --monitor" notices that a check has finished it looks to see if
there were mismatches.  If there were it sends email about it.  This should
happen when the check finished because it came to the end of the array, or 
when the check finished because it was stopped after 6 hours.

"mdadm --monitor" is run by the "mdmonitor.service" systemd service, and should
be started whenever any mdarray is started.

Do you have mdmonitor.service running?

Do you get mail sent to the MDADM_MAIL address in /etc/sysconfig/mdadm?