Sometimes speed and duplex settings of the data interfaces on the VMX nodes can lead to misunderstanding.
FortiGate-VMX (global) # diagnose hardware deviceinfo nic internalName: internalDriver: tunVersion: 1.6FW version: N/ABus: tapHwaddr: 02:69:65:d1:60:52State: upLink: upMtu: 65521Supported:Advertised:Speed: 10full
It is expected behavior.
Data interfaces are virtual interfaces, therefore the speed/duplex definition of a regular physical interface is not applicable to the virtual interface.
The actual speed of the virtual data interfaces depends on the physical layer of the hypervisor.