Page 452 of 7.2 study guide, specifically states "Although you can use the sniffer command to capture traffic on switch ports, the types of packets capture by the sniffer are very limited.
The use of the sniffer command on FortiSwitch CLI can be unreliable on port 23 for specific reasons related to the nature of traffic on the port:
Reference:
For guidelines on how to properly use sniffer commands on trunk ports and configure VLAN filtering, consult the FortiSwitch CLI reference available through Fortinet support channels, including the Fortinet Knowledge Base.