As you can see, it has got new growth however the lower leaves have started going brown/ curling up. it was planted 3 months ago. It gets about 5 hours of direct sunshine .
It looks like it is either being scorched by the sun, or getting wind burn. They prefer a shadier, more sheltered position.
The ones with dissected leaves are also more easily affected by weather conditions. The reds can tolerate quite a lot of sun once established, and it actually improves their colouring, especially if they're the palmatum types, but 5 hours of sun at this time of year and through summer, is a lot for a young plant. There's also big differences with the strength of the sun depending on the location you're in, so it always helps if you can add your location to your details Has it got enough moisture in that site? It takes a while to get them established, and if you're having drier weather, you need to ensure it gets well watered over the next few months. They need good moisture but also good drainage.
I agree with the above, all those rocks will also hold and radiate a lot of heat which isn't ideal. Japanese Acers are understorey plants, they thrive in sheltered semi shaded conditions in their natural setting. But a lot of watering can help with more sun exposure from experience of growing them in large pots.
