This article describes the issue where FSSO events are not collected by FortiGate after the upgrade of FortiAuthenticator to 6.4.5.
Solution: Disable Enable encryption feature
FortiAuthenticator now offers a server-side TLS support option so that FortiGate as an FSSO client can be configured to connect to FortiAuthenticator over a TLS connection, and this is enabled by default after the upgrade.
Step: Disable this by moving the toggle off the Enable encryption option under Fortinet SSO Methods > SSO > General.