Not only is energy divided in spring, but weather is getting hotter, worse, and as many of us know, early heat can be just as, or even more fatal then a late frost.
Look at this Latitude-Time chart of the intensity and duration of sunlight. At Detroit's 42.6° latitude April 1st is just beginning intense sunlight, and August 1st intensity is high but diminishing. I store & start all my JM in full sun and move them to partial sun by June 1st. They like sun, but intensity and duration matters greatly. The intensity and duration of sunlight available from August 1st-on is just the kind that mature leaves need, not too intense and diminishing, and not too long and diminishing. The intensity in August is about the same as April/May, a period where we get great growth, but no crispyizing of edges.Sure you expose the canopy to full sun after late-summer repotting. Temperatures will go down quickly , days are shorter. The full sun on mature foliage will create energy to get lots of new roots quickly. The evaporation is there, but it's getting better every day.