Good Morning,
I have a Sharp’s Pygmy that had a strong spring flush of growth, then suddenly died back when temperatures warmed up.
I have a vague memory of Ryan discussing this in one of his videos, but haven’t been able to find it. Something about actions taken the prior fall or during the winter cause can cause this to happen.
Thanks,
Ken
Think I figured it out. It was sitting in water even though the top 1/2 inch was dry.
Pulled the top inch of soil off and it was water logged. Went down a little further and the soil, had broken down, could make kind of like clay balls with it 
. Was using 1/3 potting mix, 1/3 pine bark fines (sifted), 1/3 surface (sifted).
Repotted (in July) with 2 parts pine bark fines, 1 part diatom earth (oil dry), 1 part part Turface, all sifted and a layer of sphagnum moss on the surface. I cleaned out as much of the nursery mix as is dare, and watered it well with Seaweed extract. With in a week it started pushing new growth.
Had it receiving morning sun and dappled sun the rest of the day, moved it back a bit to receive less direct sun.
This should have been under Beginner, not here.
Over watered the trees as the temperature rose. Cause root rot and they go down hill fast. Japanese Maples like loose damp soil.
Why did you not remove all the nursery mix? I hose off everything when I repot from nursery mix or akadama when repotting deciduous.