As per the description, the FGT cluster is in a split-brain scenario.
A split-brain scenario is usually caused by a complete loss of the heartbeat link or links. This can be a physical connectivity issue, or less commonly, something blocking the heartbeat packets between the HA members. Another cause is congestion and latency in the heartbeat links that exceed the heartbeat lost intervals and thresholds.
To resolve a split-brain scenario:
>> Be physically on-site with the FortiGates (recommended). If this is not possible, connect to the FortiGates using console access. >> Identify the heartbeat ports, and verify that they are physically connected and up. >> Verify that heartbeat packets are being sent and received on the heartbeat ports. >> Verify that the HA configurations match between the HA members. The HA mode, group-name, group-id, and password settings should be the same. Different >> group-id values will result in different virtual MAC addresses, which might not cause a MAC conflict. However, an IP conflict can still occur. >> If everything seems to be in working order, run get system ha status to verify that HA has formed successfully.
To avoid a split-brain scenario:
>> In a two-member HA configuration, use back-to-back links for the heartbeat interface instead of connecting through a switch. >> Use redundant HA heartbeat interfaces.
You can use the debug commands to check heartbeat communication and sync status.
Collect the output of the below command from both the unit:-
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