Look carefully at the paragraph style of the Chapter Sub-title and you probably have an attribute like Keep Options > Start Paragraph which is causing text to jump to the next page or next even page or something like that.
Normally, this would cause a new page to be created but you also have set up your pages is an unusual way which is "lumping" the pages together rather than using Facing Pages.
Thanks, Steve. Not sure about this. My document is set with Facing Pages and Primary Text Frame both checked. If I can, I'll attach a screenshot of the Style Options section you mentioned, maybe that will shed light.
Note though, that all my text is already flowed into the document and I'm simply styling one-line subtitles (because I haven't been able to figure how to properly nest paragraph styles! - that's another discussion entirely), and that the pages are being added onto the last spread in the document. For example, I'll style a line of text on Page 182, next thing I know I get the error message and see there are 10 new blank pages laid out like a spread next to the last page spread in the doc.
It's because you turned off page shuffling, so the additional pages that are added are being added to the last existing spread, and there's a limit of ten pages per spread.
I just turned it back on (Allow pages to shuffle) and so far so good. I swear I had the opposite problem early on, which is when I was told to uncheck that box. This is rather confusing, but if it works, I'll go with it. Thank you!!
(I would like to understand it better though - why some people say to uncheck it, others to keep it checked... and why it would add pages to a SPREAD, as opposed to just adding pages to the end of the document as a new facing page spread.) For now, MUCH APPRECIATED!