Pete
I have experienced the same "sun burn" issue here in MD on newly established plants. I have about six trees, mostly HC, that I started early on from cuttings. From rooted cuttings, to cups, to one gallon pots. These plants are well acclimated humidity wize, but the last few days when I had them in direct sun, their bright dark leaves have paled. So, for sure too much sun. i left two test plants in doppled sunlight, in our sunroom, no change. The direct sun just too intense on those placed outside. BTW, although I said these are new, the plants themselves are about a foot tall with a good 10-12 leaves. Just big babies!