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 193223
The 'srcname’ parameter in the traffic logs comes from the unit detection/identification feature (enabled at the source interface) and the scan does not request periodical updates from the source what are the current information (once the data from the host are obtained once they remains within the FortiOS).

To update the source name, clear the record from the unit inventory and let the unit be newly discovered:
# diag user device del <MAC address of the host>
To clear all discovered records/hosts:
# diag user device clear
Alternatively, to achieve a periodical update, it is possible to schedule a system auto-script feature:
# config system auto-script
    edit "devices_update"             <----- Type whatever name you would find suitable.
        set interval 86400            <----- Frequency on how often the script would execute.
        set repeat 0                  <----- Repeat infinitely.
        set script "diag user device clear"