Hello Andrew,
Many thanks for taking the time to go through and understand my query. Your reply seemed like exactly what was needed, and I have been trying to implement it. Ultimately I did not get success, as I could not successfully drag the section break.

But I took a different route, which did solve the problem: I copied all the content of another chapter which was correctly formatted, then did a next page break at the end of chapter 11, created a new chapter following it, then copied all the content from page 11 and pasted it over the chapter I had just created, thus creating a new chapter 11. I then deleted the old, defective chapter 11 and everything is now working perfectly! It was perhaps the longer pathway, but did yield the needed result. Again, many thanks Andrew-- I will try your suggestion again the next time this happens.