When upgrading to the latest UniFi Controller and AP firmware, RADIUS authentication may stop working in that the wireless SSIDs that require RADIUS authentication will not even broadcast.

If this happens to you, check that there is a password set in the RADIUS Accounting Server field:

UniFi Controller > Settings > Profiles > Edit the respective profile

It seems that this field is left blank by default when upgrading to the latest version of the UniFi Controller. After filling in this field with the same password used above (clicking on the field will display the Password/Shared Secret), the wireless SSID immediately started working again.