Wilted Copper beech

If anyone had an experience with sudden wilting of a beech or other deciduous tree and found the reason please keep reading. Being minful here to not waste anybody’s time :wink:
My copper beech is ‘mine’ 4th season. It is a starter tree ab 40cm high doing well so far in ordinary deep black plastic container. What happened this year was it leafed out very late and leaves vere green until sun rays turned them purple. This is first time and I sensed this might be sign of a weakness. I know Copper Beech is natural mutant of a common beech so it can have green leaves esp in bottom part of a canopy due to luck of direct sun light but this isn’t the case.
It was repotted this year with minimal root work cos I want nothing but it to grow atm. Same container, same soil mix from one of the best bonsai specialists in UK but I mixed in more akadama (50:50)
It has been moderately fertilized with fertilizer Im always using along with other trees month ago (some leaves were already out)
I noticed the wilting at least couple of weeks ago and it had very slow process. Today it looks as you see on pictures. What I did/going to do:

-it is now in semi shaded area facing south (I don’t have greenhouse so this is shaded by a bush and large leafy plant) instead of south facing location on full sun.
-I checked branches by bending them to see green cambium - this is ok
-it doesn’t suffer of lack of water, this I checked regularly through the time it was wilting
-tree doesn’t show any signs of disease or pest presence, leaves are soft to touch without any damage
-Im skipping fertilisation completely
-I will water only when it’s on the edge of getting too dry (baring in mind beech is thirsty tree)

Anything else you would suggest/recommend from experience or wide knowledge before I dig into the pot to see roots? This is the last thing on my list if my ‘intensive care location’ won’t improve the look of the tree.

Thank you