Hopefully someone can shed some light on the roaming issue. I do not have a roaming/mesh-like setup. Since you poll all AP's I would expect the plugin to always find the MAC in question on one of them at any time, but that does not seem to be the case here.
Regarding the USG: Have you tried a ssh session from the raspberry cli to the USG and running 'show arp' (on the USG)? Make sure you are logged on to the raspberry with the same user that is running Domoticz.
I see some post on the internet about limitations of SSH login on the USG, like: https://community.ui.com/questions/cant ... af91ed6f67
There is no 'unifi-arp' custom command in the plugin. According to your log the plugin is using wlanconfig on the AP's (probably by auto-detection).Script approaches each AP via its IP with the suffix =unifi-arp.
The suffix 'unifiusg-arp', which you wrote further on, does exist to force a 'show arp' command on the USG. I can't remember why I added that custom command. Maybe the USG does not have a standard/full Linux command set(?)The reason is probably somewhere in this thread