What can be done to stop the rusting? What design changes can be made to the fence panels to stop the rusting? Should the powder coating have fully encapsulated the edge detail?
Yes, there is indeed a reason for this rust. A likely cause for corrosion at the locations you mentioned is called the Faraday Cage effect. Powder coatings are applied electrostatically, and the charged powder particles deposit on the nearest grounded surface. They do not penetrate much into recesses where the plate and wire mesh are welded together. The same lack of coverage tends to occur at points where wires cross in the mesh. Since the powder coating is thin or missing in those spots, early rusting will be seen, as you have discovered.