Document isn't adding pages as expected; allows overset instead
I have a spread with a primary text flow on each page. They are connected (which we know, because InDesign does not allow primary flows on facing pages if they aren't).
I have a one-page document (which, not surprisingly, starts on the right page). If I insert a page break and keep typing; or if I type a bunch of extra lines and then go up and insert a page break; or if I just add enough lines to overlow the page... InDesign isn't adding a new page. It's allowing it to be overset.
Previously (including in the file I copied to start this one), InDesign added pages. Why would this suddenly cease to happen? All files are members of the same book, by the way.

