FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 288894
Description This article describes the case when all IP addresses are used as IP pools and VIPs are considered local IP addresses.
Scope FortiOS 7.0.13 and later.
For these cases, the FortiGate is considered as a destination for those IP addresses and can receive reply traffic at the application layer.
It is possible to modify this behavior by disabling the ARP reply with the command below:
config firewall vip
    edit <name>
        set arp-reply disable
The changes regarding the IP pools and VIP behavior are tracked in the KB article below and it describes how the default behavior was changed between OS versions: Technical Tip: IP pool and virtual IP behavior changes in FortiOS 6.4, 7.0, 7.2, and 7.4.