Hello @ThatWelshTrucker
First of all, thank you for your questions here. After reading your question carefully, I understand that you want to know about the update of your navigation data after joining TruckersMP is this a bug or a regular issue!
After reading your question, I will tell you that this is a regular problem, because when the game is updated, your game data has changed, so you need to re-select the garage and trailer, just like when you play the game without TrucerskMP exclusive content in single player, you need to re-modify the game data, and then return to the online mode. This is something every player goes through, a regular problem with the game, not a bug you encounter!
I hope my answer to this question will be helpful to you!
Thank you
Kind Regards
Toth.