This is expected behaviour for FortiGate Cloud:
You must purchase a subscription for each FortiGate in a high availability (HA) cluster. FortiGate Cloud handles each device separately regardless of configuration. FortiGate Cloud accepts inbound logs from each device independently and cannot detect whether connected devices are in an HA cluster. Though multiple HA clustered devices theoretically send identical logs to FortiGate Cloud, if one device stops logging or cannot reach FortiGate Cloud, the other devices do not send logs on its behalf.