That looks like wind damage to me. Japanese Acers hate the wind with a vengeance. Needs to be in semi- shade (no direct sun in middle of day), out of the wind, in a pot needs ericaceous compost and kept moist but not wet.

What size is the actual acer and the pot as the acer looks to be very small to me.

Is it in a large enough pot (hard to see the size on photo)? Acers like a moist soil and yours may not be getting enough moisture if the pot is too small. I would re-pot, using an ericaceous compost. You were right to keep it in a shaded position. Also, it doesn't like a windy position.

Those leaves, the newer ones, look slightly contorted or distorted to me. Unfortunately, the pic's not clear enough tell whether there are very small indentations or pits in the leaves as well, which eventually may turn into holes. Are there any?
And is that paler banding or spotting on the newer leaves? A photo which shows the newer leaves in focus would be very useful - at the moment, I can see your finger really, really clearly, lol!

Hi, I also have this problem. Mine is in a large poy but pretty much in full sun.
A mate of mine also had this problen in year one but not in yeah two. He's not changed position or pot!

Thanks for all your advice peoples.
I have re-potted, but I suspect the wind damage has done it for this year.
Will try all ideas for next.