cork bark elm problem

This is a feature of trees we call Spring variegated elm. Mine shows alot of this in spring that stays after the leaves have hardened off but fresh growth later in the year does not usually have it.
I figures these elms you guys have on the other side of the world are somewhat different in the way it comes about but the change from year to year is not that surprising to me. I have seen a mature trident yard tree go from single lobed leaves shaped like a fig back to the usual trident shaped leaves but this took about three years when we had some severe weather patterns. At first I thought it was some weird regression thing from being cross bred or grafted But after getting a professional ID I was told it was a natural thing for Tridents.
I am hard pressed to see much deformation in the leaves of your tree but I would treat that as a separate issue to the variegation which is now just a natural part of the tree that may come and go.

I am usually not that keen on variegated trees for bonsai, but I have come to like the speckled look of these trees.