I read the text after I saw the pictures and was thinking the same thing. Too much nitrogen or a bit to less sunlight. Also Acers don't like hard water. If your living in a chalky kind of area, try boiling the water first or use mineral water to water the tree. The tree doesn't look unhealthy though, it's just that the rather long internodes and long leaves indicate that it's not feeling completely well either. just tweak a bit and make tiny adjustments to the water/feeding program and see if it improves.