There're various reasons why this can occur, but I'd suspect difficulties with the potting medium. Miracle Gro would not be a product I'd recommend for a containerized J. maple and one that includes a moisture control product even less so :-) This mix is notorious for retaining excessive moisture and that can pose serious problems for maples.

Japanese maples like an acidic soil and one that drains very freely. A coarse textured, barky mix would be preferrable - the texture of both the MG and the cactus mix is too fine for your needs. Too fine a texture leads to aeration issues and that combined with excessive moisture can only lead to problems eventually. The gnats -- most likely fungus gnats -- are indicative of this. They tend to populate excessively damp soils. And MG soils contain their fertilizer products as well, which I also don't recommend for maples - too high a chemical salt content.

Not an ideal time to do so, but I'd seriously consider repotting in a more appropriate soil. Search the archives of this forum for discussions on a suitable mix - it's been discussed extensively. You may need to water more frequently with a better draining soil but there is far less risk of overwatering or dealing with a saturated soil with poor aeration. And there is no benefit to misting the plant if properly watered. If exposed to sun when doing this, it can also cause foliar damage.

Thank you for your intelligent reply. After reading your credentials I will search for a different potting medium. However I would have thought that the tree would have shown signs of stress before this four month period.

I agree with gardengal. The reason you are probably just now seeing signs is because in spring most maples take off with growth, then slow down as warmer temps and summer approaches. The extra moisture is not wanted now as the growth slows in the heat of summer. Make sense?
Kay