If your plant has been indoors or more shade for awhile then put out in direct sun immediately, the leaves will bleach and sun burn like that. The plant can take full sun, but our sun conditions here is different, too strong, so it helps to put them in part shade till it gets used to our growing conditions. And try to water them often, our area is so dry, Crown of Thorns likes to drink a lot when days are too dry and hot. That should help it keep the leaves during our hot days.
