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.