“Dorico will always create more pages to fit in the rest of the music if it runs out of frames in the main chain that comes from the default master page, on the basis that if the music hasn’t yet all been laid out, you don’t want some of the music simply not to appear at all.”
This makes sense. All of the music is laid out in layout frames. The master frames created on new pages contain the same music as in the layout frames on page 1.
“You should find that Dorico doesn’t create any more additional pages to the right once all of the music in the flow(s) assigned to the layout has been displayed.”
I’ve deleted the music in the layout frames on page 1, which has now deleted the pages with master frames on them. But I still don’t understand why the music of the last few layout frames of page 1 was duplicated in master frames. I’m being dense… the lightbulb will come…!