I spoke too soon. Viena 0.992 has corrupted this later soundfont file when adding new samples. As this later file and the earlier file have both probably evolved from a common earlier one, the problem may lie there. In other words, the problem is in my soundfont files, as I have previously suggested. I am the only person reporting this, so I don't think it warrants any more attention. Fortunately I have noticed no problem in the actual use of these soundfont files.
All about the SoundFont editor Viena