Partial Coverage During Zinc Plating Bath
It is hard to imagine this would be due to an electric field or a magnetic field. 

I would say this has to be a different material or coating in the region where you are having bad coverage. 

Would it be possible to sandpaper this piece to get rid of any potential coating? Even leftover glue from some sticky on top would cause an issue.

You could also try to electrochemically clean the surface immediately prior to the plating. Applying a highly cathodic potential should lead to some electrolysis that would evolve hydrogen gas which would knock any impurities off the surface.

Either method could roughen the surface somewhat, I don't know enough about motorcycles to say if this would be a dealbreaker.