I considered all options but I think a root issue is least likely. Tree is in good mix getting good drainage. I don't think rot is a possibility and since repot was a year ago I know it's not bound. My only guess is the problem from last year I had with it carried over. The last few weeks have been relatively dry. Spotty rain, more pollen falling than I have ever seen, tree was sitting in grow box in ground.
It does sound like you have a good potting mix, but just to clarify- root ROT is certainly not the only root issue out there, and one of the least common ones, yet most OVER diagnosed Issues our trees come down with IMO. Root aphids can ravage a tree and are completely unseen until you check for them... A tree like a Maple can become pot bound very quickly when growing happily as well which can lead to stunted/ slow growth and sometimes leave problems... But of course if you just out it into this grow box last year, that is not likely an issue yet... Then there are chemicals that can wreak all sorts of havoc on roots- ferts mixed too "hot", something you treated the leaves with that dripped heavily onto the soil and got into the dirt, as Oso was implying- something that leached out of the wood... So many options.
I don't know what exactly is ailing your tree man, just saying there are lots of things that can affect the roots negatively regardless of the soil it is in.
Hopefully it isn't anything major. Japanese Maples have very delicate leaves and a I find they often show little imperfections even when almost nothing is really wrong... Just too much sun, wind, heat... The new growth gets a little too dry or something real simple like that... Then all of a sudden we are freaking out that our tree might be sick when it is still growing strong and looks fine again after a few weeks of cooler weather or whatever...
FUNGUS!!! 
