I haven't personally ever seen the 0xfeff characters InDesign uses to hold the xml tag markers trigger a recomposition.
When I've seen recompositions that didn't make sense to me, it has often turned out to be a document created in one version of InDesign, and then converted to a newer version. Touching the text in a particular story in any way triggers a recomposition of that story using the current, newer engine. Have you ruled that scenario out—by, for instance, recomposing all stories (command-option-/ on the Mac)? Just an idea.
Thanks Jeff. command-option-/ was first thing i tried. Nothing on that page reflowed. Then i added one processing instruction. Still nothing reflowed. Then i added a processing-instruction in another line in the same para. Then the para reflowed.
Since posting i also saw a separate thread on someone inserting index markers getting text reflow, and someone replying saying:
"Yes, this is a known issue. Zero width characters are not really totally
zero width. Inserting them can cause text reflow... "
Granted, they were talking about CS2 at the time, but at least it demonstrates a precedent that backs up what i am seeing today.
"Known issue" in 2008, and still known to us today...
I guess I can be glad I don't often work with long documents. Maybe someone who does will come along with a workaround...