FortiOS6.0.4 Every port starts sudden timeout and stop access

Hello,

after working a day my FG1000D suddenly starts detecting session timeout at port13.

and a few minutes later all the port became to start detecting session timeout and stop access, but not failover.

every port didn't detect link down and also didn't reboot.

it looks like the session stucked without errors.

after pulling out the lan cable(HA monitoring) it started to failover and worked!

i had a maintenance replacement and failback the machine it also worked!!

is anyone having the same problem!?

or know if theres a bug....

HA(set override enable)

FortiGate1000D ×2

FortiOS6.0.4