The ignition is generally separate from the headlight switch. As Bill says, you have something wired wrong. There should be no connection between ignition and headlight switch. Or coil and headlight switch. Get a 1950 wiring diagram and follow every wire from/to the headlight switch. If that's OK. Report.

Also inspect the horn relay circuits and ignition lamp wiring.

Somehow you are disconnecting the coil power. Something very goofy. Something being tuned off.

If something is being shorted out, it's not fuse protected.

I've read your other post and am not sure of the present situation.

1. Does motor start and run fine with light off?
2. Does motor only die on full headlight out position? Runs on half out position?
3. Does ammeter seem normal reading while running?
4. Any other problems?
5. Battery voltage reads per spec at rest and at high idle?
6. The present issue is just that motor dies when H/L sw is full out?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn