NEW DEVELOPMENT OF ISSUE:
I now realise that this issue does not happen when adding chapter breaks into the main body of the book, only in the tile page, publishing details and copyright page, and dedication page. After that, the chapters start and they seem to render fine. So... by using the same action - clicking to add Chapter Break where text ends on a page - throughout the document, the first 3 pages end up having the left-alignment-impossible-to-center-again issue, whereas all the rest seem to be fine.
I've now tried files from a variety of sources including several different settings variations when creating the basic ePub file out of IDCS5.
Stranger and stranger it is becoming...
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