Okay. It was not clear that you were working from a Book structure ('book file' can be ambiguous).
The solution remains the same, but it does open another avenue of testing. First, try exporting each component INDD file to see if the rasterized content appears, and with which ones.
Then do the INDD-IDML-new INDD purge with any that do produce faulty content, or all of them. There's really no second fix I can think of for this. And since you're there already, create a new Book structure to make sure that feature isn't dragging in some kind of corruption (although it's pretty passive, on top of the individual component INDD files).
I remain baffled that a reflowable export is creating any rasterized content. However, I will ask this: within your chapter files, what's there that is not simple flowing text content? Placed PDFs or INDD files? Odd image file formats? Text boxes with highly formatted content? Any of those might trip up the export process and come out the other end as rasterized content.
Ok, so here's what I had to do. I went through and rebuilt the entire chapter from scratch and that did it. Something was off about the previous Chapter 1 files. What that is, I have no idea, but rebuilding it solved the issue. Thanks for the ideas everyone.