What you've stated is what I presumed just based upon the sequence of events during my experimentation.
This makes me wonder whether Mr. Cavanaugh has ever run the Adobe Accessibility Wizard in Reader DC (or even in Acrobat) or may need to do so again if something has gone awry.
If the answer is that the Accessibility Wizard has been run, then I'd try restarting NVDA with add-ons disabled to see if an add-on may be the root of the "not behaving like a web form" behavior for a PDF fillable form.
Brian - Windows 10, 64-Bit, Version 21H2, Build 19044
You can't crush ideas by suppressing them. You can only crush them by ignoring them.