I was doing some limit testing in Drowned Lands hauling a Azov 5 on a heavy-duty low boy trailer being pulled by a Freightliner M916A1. When the M916 got stuck (Location of Azov on screenshot below), I switched to the Azov and backed it off the trailer and used it to winch the M916 back. When I released the winch, I watched the M916 begin to drive off on its own until it went behind a shaded area of the map that I had not discovered yet. This caused both the truck and trailer to disappear off the "Trucks & Trailers" list. I watched this unfold live on the map. When I drove into the swamp, the M916 was stuck but still trying to drive forward. Once close enough the truck and trailer re-populated onto my "Trucks & Trailers" list and I could select it. Once switched over, the truck stopped moving and everything was back to normal.