Something weird has been happening in 16.1 (Mac OS 11.2.3). I've never seen this before and now it is happening a lot.
Minor edits--things like adding a word; adding a space; changing URL line breaks in a footnote--cause the text to reflow, skipping the next page completely.
The weirdest thing is that I can fix the problem immediately by removing overrides on the paragraph that precedes the break. But the paragraph doesn't have anything to override! And yet option-clicking the paragraph style fixes the text flow.
The only thing different about this book, compared to others I've laid out, is that I'm allowing footnotes to be split.