I am having a similar issue to my tractor and have traced it to the electrical system not operating the fuel shutoff solenoid when the starter is engaged. Started out as an intermittent issue but became a permanent problem.
I jumpered the solenoid right to the battery and fires right up. Once it is running the normal feed to the solenoid keeps it open. (note you have to remove the jumper or nothing shuts the tractor down, including the safety switches)
I have not traced it to the specific component that is not working properly but here is what I did find.
Battery is at 12.8 V when key is on. 14 V when tractor is running.
Drops to 10.7 V when cranking but that does not seem odd given the oomf the diesel needs to turn over.
but....
I'm only getting 9.6 V at the key switch when cranking and 8.7 to 9 going out of the key switch. My guess is this is not enough for the "fuel control unit" or the fuel shut off solenoid to open. But need a wiring diagram to go further.
For now I'm putting in a jumper switch to power the shutoff solenoid directly from the battery to start the tractor.