I found that any time we install an Apple security or OS update, then the Mac will fail to auth to 802.1x with System Mode (computername). Once we connected to hard wire, or vpn, then the 802.1x will work fine.
We use Cisco ISE for our wireless back end. We found out that the mac were passing the "username" with the wrong prefix. it was trying to use the first part of the FQDN. i.e. XXXX\computername (but it should be DOMAINname\computername)
Our network team found a feature on the ISE that allows you to create a rule to re-write the string so it matches what ISE expects it to look like. When the Mac sends FQDN\hostname, we are now re-writing it to DOMAINname\hostname. This has resolved the issue.
Our Apple support rep still has our case open and it's with Engineering for now. He said that if more people with the same issue were to open cases with Apple, that would raise the priority of getting this fixed. If you want open a support case, you can reference my case number. 101436764941 is the Apple Support case number.