Thanks again. Just checked out both sites and StackOverflow has a little more recent questions related to OpenXml Pivot Table but not much discussion/answering. Will check that anyway as one can also find something from questions or posted code.
Regarding my original question, I re-watched the video from Bob/Eric, and found the root cause: mishandled relationship id when adding pivot cache entry to the workbook Xml file, partly due to the fact I didn’t know there are multiple .rel files related to a document/package (under the parent node, like a workbook or worksheet).