I am in doubt that there is a loop or IP conflict.
If you configure VLAN-tagged interfaces on FortiGate, the untagged traffic received by the interface will be in VLAN-1. If FortiGate has another port in VLAN-1, it will make a loop.
Could the PC ping Fortigate and the FortiGate ping the PC?
Check the arp on both to confirm the MAC.
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