Can you confirm that data.narrativeId is not null for the missing document? An entry will not be written to the Index if all of the terms resolve to null, or if all of the values resolve to null.
Thank you for checking on the document history, which should rule out the delay due to caching. (note: stay tuned to the link to see progress on that).
You can also test if something is wrong by creating a new index with the same definition and see if it behaves differently from the existing one.