The article recommends that the HA management IP address is not used for managing FortiGate cluster over FortiManager.Solution
Do not use the "ha-mgmt-interface" IP address for adding FortiGate cluster to FortiManager as managed device. The HA management IP address is unique for each cluster member. If the FortiGate cluster performs failover to the secondary cluster unit, the FortiGate will not be manageable from FortiManager. .
FMG --- FortiGate_HA_Cluster
config system ha
set ha-mgmt-interface "port1"