The TYM T264 does not have a seat switch, like my mower. While looking for the switch, I noticed that the following actions will shut the engine (3 cyl diesel):
-Braking
-Turning lights on
-PTO (electric engagement)
Starting requires you to depress the brake and release as soon as the engine turns over.
Any thoughts
In that case, you must have a short somewhere that is causing the fuel shut off solenoid to shut the engine down. Not much we can help other than suggest the you try to inspect the wiring on the tractor for any damage, pinch points, etc.