While I have not discovered the source of the problem, I have solved the annoyance issue.
The spreadsheet that I was working on was a 'clone' of previously used spreadsheets - I create a complete copy of last year's spreadsheet at the beginning of each year. Likely, the resulting spreadsheet was at least five years old and possibly more. It was a complex document with many different sheets, formulae and tables. Inevitably I suspect, there was a build-up of 'cruft' and probably some errors.
I exported all the complete document to an .xlsx file and then imported it into Numbers. (I was impressed by the excellence of organization in the resulting file with its Export Summary and multiple sheets) I have now rebuilt the document as much as was necessary to reflect the utility of the original. No problem now with auto-save.
The good folk at Apple's Numbers support contacted me to obtain a copy of the original to try and source the problem. I declined for reasons of privacy.