Re: NVDA and Excel problem
I'd check on NVDA's GitHub Issue Page to see if this has already been reported and, if not, report it. Tossing something like, "Excel 2016," or, "Excel 2016 dropdown," in the Filters edit box, after the already pre-populated "is:issue is:open," should find you any issues related to Excel 2016 in general or Excel 2016 and dropdown boxes in particular. If the issue already exists, I'd add a comment to it stating exactly what you've said here. If it doesn't, then opening a new issue as a bug report is the way to go.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Science has become just another voice in the room; it has lost its platform. Now, you simply declare your own truth.
~ Dr. Paul A. Offit, in New York Times article, How Anti-Vaccine Sentiment Took Hold in the United States, September 23, 2019