(This is on macOS Apple Silicon, so it might not affect other platforms.)
Figured it out. Apparently the PDF bug(s) in previous 3.x versions horked the PDFs in the Presentations database, and 3.1.7 won't even display the Presentations pane if there are horked PDFs in the database. Since I didn't have a Presentations pane, I imported a PDF as an Image, so it only pulled in the first page.
The "fix" for the Presentations pane is to disable everything in the Presentations preferences, and restart OpenLP. The Presentations pane should now be visible, and you can delete every presentation in there -- or at least every PDF presentation. Then re-enable whatever you need in the Presentations prefs (PDF, Keynote, allow override -- but DO NOT enable Impress!), and restart OpenLP. You should now be able to import PDF presentations and have them stored properly. Fingers crossed.