Fortigate 601E - HA (A-P) session pickup failure when failover (v6.4.9)

Hi Gents,

we set up 2 fortigate 601E running in HA (A-P) mode, i.e FW01-Primary, FW02-Secondary. the system is configured in transparent mode, with one uplink and one downlink port. Both ports are set as monitor ports in HA settings. I expected in the scenario when hardware fault or link fault, failover will be triggered and session will be pickup automatically. We run multiple tests for HA features.

Observation #1: #When we reboot the FW01, FW02 becomes the primary and pickup the sessions. However, when FW01 was bootup, FW01 was taken over as primary but the sessions were lost. As long as I reboot the FW02, the sessions can then be recovered

Observation #2: the uplink ports (connected to a switch) is one of the monitor port for failover. When I shutdown the uplink port at switch side, the failover does work to FW02 as primary. However, when I turn on the uplink port, FW01 resume to primary role, while sessions are lost. I can only shutdown the 2nd uplink port connecting to FW02 or reboot FW02 to resume all the sessions.

Does anyone have suggestions to me ?