fortinet FGT40F - ping package loss in the IP-sec tunnel

the image you show proves there is ping loss. Not in the tunnel, not in the Fortigate - can simply be between 2 servers on lan. To troubleshoot this, contact Fortinet TAC support, or follow IPSEC troubleshooting guides. Here are some articles you can follow:
https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-IPsec-flapping-or-packet-loss-after-upgrade/ta-p/284890

https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-Troubleshooting-IPsec-Site-to-Site-Tunnel/ta-p/195672