Would you share a small sample of your Excel workbook? Combining it with Superstore sales as an example workbook will provide information not readily present through your question.
Thanks very much for the reply, Brad! I appreciate your willingness to help.
Your reply did actually resolve the issue for me, believe it or not, though it didn't do it in a way that will help others who may have this specific issue in the future. In editing my dataset to remove personally identifying and proprietary data (to be able to share it), I found that the issue "went away". My original workbook still was experiencing it, but when I started "from scratch" (same data, but new Tableau workbook), I was able to accomplish the territory creation / geographic data definition I was trying to do in the first place.
Unfortunately, I don't have time to recreate everything I'd done with the workbook to recreate the issue and isolate the problem, so I'm going to just call this "solved" for now and wait to delve deeper until it happens again. Kicking the can down the road in the name of pragmatism, I'm afraid ...
Thanks again, so much, for your reply and offer to help. I really appreciate it. Cheers!