It's a bug with Fortinet (idk how they'd miss this bug, seems sloppy work)
Wildcard Remote-admin logon in browser with customized password gets FGT message "....uses default password"
We have to wait for firmware 5.6.4 to solve the BUG ID 0456638
In this moment the BUG ID 0456638 is scheduled to resolve in Firmware version 5.6.4
Update will be released around end of May 2018.