Thanks Pranita for your responding,
I collected a little information from my customer so I shared with you as below:
When traffic stopped working after disabling the monitor, do you see the IPSEC SA as Active or Inactive in the get sa?
=> My customer responses "A/I"
Who is the initiator for the VPN: ISG or Fortigate?
=> How to check it on ISG?
Please share the output for 'get log event type 536' to see if the VPN has gone down.
=> I checked the log file output of the command "get event" but I do not see anything related to VPN down of this tunnel, I think because of unset monitor vpn in configuration.
Could you share for me some ideas how to check this case, I really stuck with it.
Appriciated,