Time passes and I revisit the katsura trees. Thank you to David and Nadia for their comments about the similar sounding symptoms they’ve had. I’m none the wiser, sadly! According to Strouts & Winter frost damage from late or spring frosts can account for problems particularly on thin, vulnerable stems. That would fit, I suppose, except only one of my sapling katsuras was affected, the other, almost identical one, seems unaffected.
But a closer examination of my own sickly katsura held a surprise. Scratching the bark in several places with my fingernail showed a bright green sapwood underneath. Next spring should be interesting.
I’ve uploaded some fairly large images (click on the thumbnails below) of the living and (apparently not so) dead trees, and full size images can be found in my fell and forest gallery.