Hi Douglas42Adams,
That symptom doesn't sound like the typical USB redirection issues, that usually boil down to "USB redirection does not work at logon, but works after reconnecting to an existing session".
As for the Horizon agent's WaitForSessionIdTimeout workaround that KjellO mentioned: that would only be applicable in case you get those <MessageFrameWorkDispatch> [ws_vhub] Can't get a valid sessionId within 60 seconds log messages.
And, if you would like to configure that setting, you would indeed not be able to do that via UEM itself. For two reasons: it's an HKLM setting (so UEM can't apply it), but even if it could, it would be too late – for the Horizon agent to pick up this setting, it needs to be there before UEM even gets to run at logon.