Hi John,
And here I was thinking I was the only one who had noticed and was bothered by this problem over the years. 
Unfortunately, PageUp/Dn in Scrivenings mode has never worked right on Windows, back to the days of Windows Scrivener v1. 
Here is a discussion with Keith Blount in October 2017 where I raised it and he assured me that feature was on the list to address with the Windows beta.
I eventually reported it as a beta bug in May 2018, but it was never corrected.
The reason I’m raising this ancient history is so you know that 1) the Windows developers are well aware of it, and 2) it must be impossible or nearly impossible to solve on the Windows side. Maybe something to do with how the QT framework displays text files ? 
My assumption is, if they could fix it, they would have done so by now. And I think you and I are the only people who have complained about it on the forum, at least that I can recall. So it doesn’t seem to be impacting a huge and vocal population.
It was a real pain for me when I was keyboard only, but it’s less of an issue now since I’ve gone to a trackball. However, I would certainly use Scrivenings mode more frequently if PageUp/Dn worked correctly.
Best,
Jim