I don't have a UDS but I've seen something similar in my WSM at about the same 5 hour point. Early on I would carefully pack my fuel because that's what I read. No longer. Now load my fuel by dumping.
Why the temp drop? As lump and briquettes burn two things happen; they develop an insulating layer of ash plus they compact. The ash happens more so with briquettes than lump, but both still ash over. Both ash and compaction cut down on the airflow around the surface of the hot coals regardless of vent settings. The insulating ash and the natural compaction of the fuel reduces the surface area of the fuel exposed to air that can burn.
In my WSM when that happened I have a long set of tongs I used to "GENTLY" stir the burning fuel to knock the insulating ash off the fuel, instantly providing more surface area that can burn. I could get another 90 minutes or so of steady temps without adding any more fuel before the temp starts dropping and it's time to stir again.
The point is air feeds burning fuel, and it is the surface area of the fuel that must be exposed. The main reason lump burns hotter than briquettes is due to more air space in the irregular shape of the fuel load, exposing more surface area. I mostly use identical sized stamped briquettes though so to maximize the air space in the fuel I just dump them in haphazardly and bury wood chunks in the fuel, maximizing exposed surface area.
I now have an air pump installed and that fire can burn for hours and hours without any stirring at all. 8.5 hours is the longest I've gone so far since installing the pump. The pump provides just the right amount of air to keep the set chamber temp stable and the fuel burning efficiently.