You have configured the performance SLA with the probe mode as Prefer Passive.

FortiGate passively monitors the member if TCP traffic is passing through the member.

After FortiGate switches to active mode, the SLA performance rule falls back to passive monitoring after 3 minutes.

FortiGate passively monitors the member if ICMP traffic is passing through the member.

During passive monitoring, the SLA performance rule cannot detect dead members.

FortiGate can offload the traffic that is subject to passive monitoring to hardware.