This issue, I believe, has entirely to do with the configuration of the PDF reader,
and nothing whatever to do with Scribus.
I am using Foxit reader mostly, and it has a View menu which gives options for
viewing single pages, single pages scrollable, and double pages beginning with
either a right or a left page. When I export PDFs from Scribus, they always show
up in Foxit reader by default as single unscrollable pages, and I have to set them
to open as facing pages with a right page starting, requiring two clicks. After that
Foxit remembers the setting for that PDF file.
Scribus's two-page layout on facing pages, with right and left master pages has
been working very well for me and I haven't had any problems except the page
number print issue which I have worked around. However, the deal is that Scribus
still exports the pages one at a time. They are correctly printed for right and left
pages, but the PDF viewer just sees one page at a time, and it is perfectly possible
to view the two-page spread reversed by changing the setting in the viewer.
In other words, Scribus's two-page spread only gives you a right and left layout on the
pages; it appears that it cannot, or does not, actually tell the PDF viewer how to view the page.
Of course there is a setting in Scribus's PDF output dialogue where you can instruct
the Viewer to view the file as single page, double page right, or double left,
but it doesn't appear to me that it works. At least Foxit doesn't see it.
PDF readers may be different.
I am using Scribus 1.5.2.
jack