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.
When application control is used, replacement messages for some specific websites are not received.
Go to developer options in google chrome using 'Shift+Ctrl+I' and check the TLS version of websites like 'Youtube' or 'Facebook'.
It is normally version 1.3 for Google Chrome and version 1.2 for Internet Explorer.

As TLSv1.3 is not supported for inspection completely in FortiOS v6.0 or below, any website using version 1.3 will not processed by the FortiOS v6.0 or below properly causing no replacement message for them.
On the other hand Internet Explorer only use TLSv1.2 so replacement message for all website in Internet Explorer will be visible.

Upgrade the FortiOS to v6.2 or Above is the solution.

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