we have 9 posts on Prod which have no featured image and they are all stories, but from all kind of dates.

I see no pattern except the fact that 5 of them don`t correspond tags with categories (they have an extra category which is not correlated with any tag). however, not sure it is because of that. 
here are the articles:
https://www.greenpeace.org/international/story/7237/food-movement-on-the-march/
https://www.greenpeace.org/international/story/7437/us-pharmacy-giant-making-wrong-choice-for-the-boreal-forest/
https://www.greenpeace.org/international/story/7056/a-record-breaking-dress-exchange-one-event-in-over-40-cities/
https://www.greenpeace.org/international/story/7368/ice-loss-the-beauty-of-the-arctic-and-the-threat-of-fishing-fleets/
https://www.greenpeace.org/international/story/7209/1000-art-works-and-counting-for-arctic-protection/
https://www.greenpeace.org/international/story/7409/5-helpful-vegetarian-diet-tips-for-meat-free-newbies/
https://www.greenpeace.org/international/story/7341/at-8-million-strong-the-arctic-story-is-just-beginning/
https://www.greenpeace.org/international/story/7314/citizen-science-in-action-open-source-air-pollution-monitoring-in-bulgaria/
https://www.greenpeace.org/international/story/7157/i-dont-want-to-be-a-part-of-the-problem/

On the Dev site - there are many more, but interestingly enough, none of the most recent has a featured image (even though there are images inside the article)