This appears to be a truckersmp multiplayer bug.
To Recreate:
Boot up into the truckersmp gameplay as usual, then go into Pause Menu, then click the Truck Dealers icon, then select a truck dealer from the map, then click 'Visit Truck Dealer' and then click yes.
Does it happen in single player and convoy?
Actions recreated in vanilla single player and convoys ETS2 with no issue.