The problem is in the switch itself, there are two contacts inside the switch for high beam one can work while the other is inop .
You need to open up the switch, but there are no parts available for the switch, so if something is broken, etc ..., yuu will need a replacement, or wire in a different switch .
I've had two switches go bad in the time I've had my '81 R65, 33 years .
Never had any success in repairing them the plastic parts inside usually were falling apart, non-repairable .
The head light circuit is not protected by a fuse .