A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
Here's how I solved my problem, and as you'll quickly see, it's not really a solution. Rather, it's a work-around that I would recommend to anyone experiencing the same or a similar problem with the OneNote UWP.
The key is: You must continue to use the Office365 version of OneNote. The UWP version of OneNote is not yet ready for day-to-day use. I have confidence that it will be eventually, but not now.
First: Backup OneNote 2016 to your local computer. This is only a precaution.
File > Options > Save and Backup > Backup
Then, uninstall OneNote (the UWP:)
Start > Right-click on OneNote > Uninstall
Restart your computer, then re-install OneNote (the UWP) from Microsoft Store.
When OneNote launches, all your Notebooks should be there and synced. It will take a few minutes for all your Notebooks to appear, so be patient.