I don't think the mulch is the problem. Both dry roots and soggy wet roots can cause such symptoms. Since the tree was just planted this past spring the root system is still very confined and thus more prone to problems with soil moisture extremes. Try digging down a few inches deep around the tree to feel the soil and assess whether it is dry or still very wet. This is the best guide for how often you need to water.
Hopefully the tree is just adjusting to the summer and getting a root system established and will turn around and do better soon.
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