Yikes, self running machine, eh?
One thing to look at is you can have that condition when all of the parts are okay, but the battery condition is low. So that is my first question: what is the state of charge on the battery? If is is low, recharge, test it independently, and then try it in your machine.
For some reason at low battery voltage, contact points in the starter relay weld together.
Happens often on Harleys, which feature high cranking amps.
On a car, when this condition happens, one could tap on the relay and cause disengagement, but the relay is rarely accessible on a bike.
When you are all said and done, there is exposure of your starter relay, from all of this contact point welding. But most of the time the relay is fine.
Very interesting failure.
Battery is where you start.

Cheers,

Doc