@freddy Not sure I'll be able to have the original trigger now that a media proxy got introduced (don't know if it works retroactively).
But I have an idea for a repro, adding then deleting an <img> to the document. I guess what makes debugging this difficult is that you need a point-in-time document snapshot to see the true source. Even seeing the <img> may not be meaningful without e.g. it's surrounding <div> with the post content.