This could be either a fungal problem, or very likely a parasite called scale. If scale, you can address it by washing the leaf with a soapy mixture like dish soap. Lady bugs can help. However you really need to take a leaf to your local arborist to see if this is fungus or scale.
A hot summer makes the trees too exposed to sun with brown leaves ,lack of water, too much fertilizer, or even some physical damage to the root system. some pests may dig the leaves and some fungus affect the leaves and make hole in the leaves and finall it falls.
Sounds like a disease. If you can, take a sample of the leaves to a nursery so they can properly identify what is going on with your plant. From there they can tell you how to correct the issue properly.
Japanese maples are very sensitive. If there are holes, it's an insect. It needs to be sprayed by yourself or an arborist. If you're spraying, make sure you follow directions.
You may have some type of insect or slug eating your leaves. Possibly a Japanese beetle? Try seeing if you can find anything on your tree. Once you find it you can then treat your tree.