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 198577
This article describes when forward traffic logs are not displayed when logging is enabled in the policy.

Firewall memory logging severity is set to warning to reduce the amount of logs written to memory by default.

Because of that, the traffic logs will not be displayed in the 'Forward logs'.

The severity needs to set to 'Information' to view traffic logs form memory.

To view the current settings .
# config log memory filter

(filter) # show full-configuration
# config log memory filter

    set severity warning                        <-----
    set forward-traffic enable
    set local-traffic disable
    set multicast-traffic enable
    set sniffer-traffic enable
    set anomaly enable
    set voip enable
    set dns enable
    set ssh enable
    set ssl enable
    set cifs enable
    set filter ''
    set filter-type include
Modify the severity  to information.
#config log memory filter
    set severity information
Once modified, Traffic logs should be displayed in the 'Forward Traffic' under memory logs.

Starting from FortiOS 6.4.0, the default severity is set to 'information'.

So Traffic logs are displayed by default from FortiOS 6.4.0.

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