Hi, I donīt have much experience with japanese maples, but those burns seems to me a lack of potassium, the leafs show the typical potassium deficiency, tips of leaves burnt in a "V"-shapped pattern, that burnt, tend to go all the way to the sides.
www.omafra.gov.on.ca/.../apple_disordersf37.htm
I donīt know if that is a common problem in japanese maples, but I think that could be also an explanation for the problem.
Regards,
Napo