I am also seeing this same behavior on FG60F after upgrading to 7.0.5 and a Fortiswitch running 7.0.3. I have also noticed that some devices will show under Fortiview Sources but not in DHCP monitor at all.
As my colleague already mentioned, this will be fixed in later release. Another workaround in case the setting of the global user-device-store-max-devices is not helping would be to graceful shutdown the wad daemon : "You can restore the user-device-store with "diag test application wad 98" then wait some seconds and user-device-store is repopulated."
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