Answer: Based on your description I suspect it is moisture stress. These trees are somewhat shallow rooted and are very sensitive to dry soil. They are also prone to heat stress, and a new tree will be particularly susceptible since it is not yet fully rooted and established.
This tree requires soil that is organic and humusy, as well as evenly moist yet well drained. This means damp like a wrung out sponge, not saturated/sopping wet or dried out. It should be watered as needed to keep the soil moist. To know if you need to water, dig into the soil with your finger. When you water, water slowly and thoroughly so it soaks down to the deepest roots. After watering, wait a few hours and then dig down to see how far the water went, sometimes it can be surprising.
Keep the mulch in a flat layer two to three inches thick over the entire root area. It should be flat, not mounded up at the trunk. Fluff it occasionally with a rake to prevent it from packing down and repelling water.
Other possible causes for leaf symptoms include overfertilization, contact with chemical sprays, and herbicide exposure. I hope this helps you trouble shoot.
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