Andrew,
I’ve almost always seen rust first starting under non-sacrificial protection at welds. I have always assumed that it was due to poor cleaning. Shop painting and hot-dip galvanizing are done after cleaning. Weld touch-up is done after slagging, so I just figured it was caused by a poorly prepared substrate. So I never looked for any researched explanation. If you see this on parts that were joined then coated, that is another thing.
HTH,
Jim Getaz