I did some further experimentation. One step that I did not mention was that I had imported into Nitro a keyword list in txt format that had been exported by digikam. Its content and hierarchy look fine in Nitro. I now notice that when I attached keywords with that list selected they did not stick, even when I went to another image and came back. Closing Nitro and reopening with that imported txt file seemed to cause the keywords to stick. They now appear in the xmp sidecar file.
Thanks, Stewart