I experienced a similar problem this weekend, except that it happened while I was sitting P2 in the main (arghhhhhhhhh!).
Digital (key start) iginition.
Engine suddenly cut out. Coasted off the track. Chain okay. Turned the key to restart and nothing. Brought it back home. Still nothing. Thought maybe I stuck a piston but the motor turns free when rotated by hand. So it is some sort of electrical problem. The battery is strong and the “ground” is there. The Positive is feeding to the key switch and also to the ignition box. There is current flowing to the starter and the bendix meshes with the ring gear and turns fine.
I have another starter so I’m going to put it in and see what happens. Don’t see how a bad starter could cause the engine to stop during a race however. I’ll report on my progress.