When I load my save game I have a helper in the tractor Class Arion 530 and goes to the farm with them.
I do not have a task.
The helper stayed on the tractor after I called him to the harvester Jaguar using the "call trailer" button.
He does not have previous task. When he reaches the endpoint, he will not disappear.
I tried to call and dismiss him again, he disappeared. I saved the game, and when I uploaded it again, he was back in the tractor.
this "Call Chaser" command (and especially the "dismiss" command) doesn't work very well - we'll probably remove it and replace it with something better. (Didn't we already remove the key binding option the controls menu?)
Anyway, a fix/workaround for your situation:
Press the Alt Key while near the misbehaving vehicle
You should see something like 64 - Claas Arion 530 written above the vehicle (and something similar above the implements connected to the vehicle). 64 is the ID of the vehicle
Open the Console
enter abortNavigation 64 (assuming the number shown above was 64) to abort the currently navigation task of that vehicle
You may have to enter abortNavigation 64 again in case there were more navigation tasks queued up and the next one starts. Just repeat it until the vehicle stops driving.