Technical Tip: Use MGMT port inside the firewall policy

The reason why the MGMT cannot be selected by the policy is because of the dedicated-to-management option. Try to remove this option and it can be selected normally by the policy:

How to fix:

config system interface
    edit “mgmt”
        unset dedicated-to <----- Unset the dedicated to management configuration.
    end

Note:

Typically based on the FortiGate model architecture, the MGMT interface will not be offloaded as they are not part of the ASIC.