This screams root issues to me and is really similar to one of my maples this summer. The soil staying wet is a sign that the roots can't draw in enough water to offset the transpiration from the leaves. The tale-tale similarity that I see are the spots starting at the base of the leaf where the petiole attaches, the same place where mine started. My tree already had weakened roots and I repotted this spring to try and correct that issue. My soil was also staying very moist with watering only needed every couple weeks, and about two weeks after I noticed the spots the tree dropped almost all of its leaves. It's now in intensive care to try and heal the roots. I would suggest trying to find a way to keep the soil from staying too wet, possibly removing the rocks you have as top dressing or tilting the tree to increase drainage. Move it to the shade as the roots can't support the transpiration from the leaves in the high temperatures, causing them to dry out and be more susceptible to sun damage. Might try misting as well, as long as it doesn't keep the soil wet.
