So far the internet hasn’t provided any definitive answers on this one. We have a 20ish year old JM. It was a seedling at a local botanical garden and very close to the plain green acers, but with better spring and fall color. Trunk diameter is about 8”. A month or two ago I noticed some relatively minor yet early leaf drop at the ends of branches. I’m in South Carolina, so it’s often Thanksgiving before they turn colors and drop leaves. I did an inspection at the time and there were no signs of insects or fungus. The leaf drop has continued and I did another inspection today. It appears that leaves begin to brown at the base, where they connect to the petiole. As that spreads the leaves eventually curl around the dead spot, die completely, then fall. Still no noticeable damage from insects or fungus. Verticillium fungus has come up as a possible culprit but I have not found anything specifically outlining this type of progression in the leaves (browning from base as opposed to tip). I will say that it has been a relatively wet summer that was preceded by a cool spring. But the tree is within the irrigation systems range so I don’t think it’s received any more water than normal. It was just free water. :-) There is a pond about 50’ from the tree.