Before v5.4, FortiOS was showing all the interfaces from a VM even when it had not an interface associated in the host. From version 5.4, FortiOS will detect on boot time how many interfaces are associated to the VM and it will list only these interfaces.
FortiGate VM and cloud VM.
In order to configure a new interface for the VM, it is necessary to power off the VM, add the new interface in the host config (VM manager, AWS config, Azure config,…) and then power on the instance.
The new interface will be detected during the boot, and it will be available in the FortiGate.
If a new interface is added with the instance running, it will not appear in the FortiGate until after the next reboot.