I have a similar problem with the Oxford Horizon Billet bar end mirrors I fitted to my Svartpilen 701 to replace the original Husky mirrors. As soon as I hit 8,000 rpm the left mirror head fell down to point at the ground. I returned it for a replacement that had the same problem. Oxford customer service advised me to return the replacement for another one as they had no history of defects with this model of mirror. The right side mirror felt stiffer to move and is functioning correctly.
Rather than try a second replacement I filled the ball joint in the centre of the mirror back with epoxy glue (Araldite) after I set the angle just right for me. The mirror still moves a bit at high speed but doesn’t fall down like it did before so my fix really has failed. I think I shall switch back to the OEM mirrors!
The problem must be the level and intensity of engine vibration at 8,000 rpm. I never stay there long enough to feel it but the mirrors don’t like it!