Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I think I figured out the problem. I have Excel & Word icons in my Dock. Every now and then I'll launch one of those apps and then notice that there is a second Excel or Word icon at the end of the Dock (in addition to the ones I placed there in another location). That happened again today. It appeared that I was running two instances of Excel, because there was a black dot under each one.
I went looking in my Applications folder and found that I had some additional copies of the Office apps. Normally I keep my Office 365/Mac Office 2016 apps inside a folder inside the Applications folder named "Microsoft Office 2016 (Mac)" (because I also have VMWare and the Windows versions of the same apps). I found that I had some 16.24 version apps inside my usual folder and some 16.25 files both inside and outside it (in the main Applications folder), so I guess the AutoUpdate program got confused by my folder organization. Anyway, I've now deleted the older versions and moved the newer versions inside my preferred folder and all seems to be OK (though after 5 seconds of testing).