FortiClient VPN doesn't apply the Username Formatting Override

I have set up a 'Credential' entry with my username, the domain, and password all specified in their relevant boxes.

My VPN requires my username be specified including the domain, however when using the FortiClient VPN Type (via FortiSSLVPNclient.exe), it doesn't specify the domain. Even if, in 'Advanced' for the VPN entry, I choose {username}@{domain} as the username format, it still just uses the {username} when connecting.

I've tested with the 'Custom' VPN type, and that does apply the {username}@{domain} override to the $VPN_USERNAME$ placeholder as you'd expect, but I do need to use the FortiClient version as the 'Custom' version has some problems with the password.