So something is wrong as the usernames should show up. Is the issue of the certificate and global catalog really right? Did you ever double-check the settings? I think it's a good way to start: https://documentation.meraki.com/General_Administration/Tools_and_Troubleshooting/Active_Directory_Issue_Resolution_Guide#Troubleshooting_Issues_where_AD_is_Integrated_with_MX_Security_Appliances_for_Applying_Policies
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