If that mac address does not authenticate anymore the data retention of XIQ Site Engine would cleanup these mac addresses after 90 days, and also remove them from groups if that option is checked in Administration/Options/Accesss Control/Data Persist...
You can view the current Meraki vsa's in control, configuration/Vendor radius attributes.How to see and add, is explained in below link.https://emc.extremenetworks.com/content/oneview/docs/control/access_control/docs/c_ov_ia_at_man_config.html?Highli...
There are so many articles written for this error, did you try to search ?One that might be a start, https://extreme-networks.my.site.com/ExtrArticleDetail?an=000077156
