The reason the shaded plants may look healthier and no rust is because the heat may stress the others and make them more susceptible to an outbreak like you said. I have observed that mine that are more shaded (not on purpose, need to trim trees) look better than the ones in full sun. Of course the ones in deep shade tend to get spindly and scapes grow too tall and no branching or buds.
These areas last year I did not spray because we were not keeping the plants anyway and they did not breakout in rust throughout the season until around September (when the fall outbreak occurs) but by this time they were being stressed because I stopped watering them for a few months.
There is more factors that contribute to rust than just moisture as you are finding out
