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.
This article describes how to check MAC-address table in Transparent mode.

In transparent mode, to forward L2 traffic, the FortiGate does actually rely on its L2 forwarding database, which can be dumped with the command '# diag netlink brctl name host root.b' (for the root VDOM), while the ARP table would only be used for it's own IP communications.

The ARP/GARP that are sent by the external units will be used to populate the L2 FDB and change appropriately the destination MAC addresses for existing sessions in case of MAC changes.

Those commands  will dump the L2 forwarding table for each VDOM bridge instance.
From there, the devices MAC addresses that are located on each VLAN/VDOM will be available.

# Config global
# diag netlink brctl name host <VDOM_name>.b
Example for the root VDOM.
#diag netlink brctl name host root.b
#diag netlink brctl list