Q.Why are Japanese Maple leaves brown and curling
Thy are drying up, curling up and falling off. I don’t see any signs of insects under them.
Certified GKH Gardening Expert
If you've had an especially hot summer, your tree may be suffering from leaf scorch. If you've had an inordinate amount of rain, it could have received too much water. Also, some insects are microscopic and their damage is all we see with the naked eye.
Here's an article that should help you identify the problem: https://www.gardeningknowhow.com/ornamental/trees/japanese-maple/japanese-maple-problems.htm