Pages Panel does not scroll to all pages in document, whether in horizontal or layout view. If panel is enlarged and the pages all fit, you can see them. If you shrink the pages to a smaller size (from jumbo to extra large) and they fit in the view panel, you can see them. But if it is a large document (more than 25 or so pages) you cannot scroll far enough to view pages past about 25 in extra large view.
Fully updated version InDesign 20.3.1. on Mac.
Every file I open is like this. Most files were created in an older version, but have been converted and saved. I have tried exporting as an idml file, but that does not work.
In the past, I have used the pages panel set to jumbo pages to see the whole document and rearrange the pages.