Conor this tree spends the entire growing season under an arbor where it receives partial direct sunlight. Japanese maple leaves are always a challenge to keep in good shape all the way until autumn arrives. Sometimes this happens no matter what. It is usually not an indication of health and does not affect the tree per se. This is one of the drawbacks with keeping these maples specially for those of us that are in more southern regions. Some cultivars such as arakawa are extremely resistant to sun and wind scorch.
So you get an idea, here is a pic of the top of the arbor where many of my maples reside in summer. I'd say trees probably get roughly 1:1 sun/shade ratio.