This is not a bug, it's an issue related to detecting your DLC.
Please go in your Steam profile -> Edit profile -> My privacy settings, and make sure that both My profile and Game details are set to Public.
Then go on TruckersMP website and go on Account -> Settings -> Linked Games / DLC, and click on Check DLC.
After those two steps, it should hopefully fix your issue with not being owner of DLC on ETS2MP.