No thumbnail in timeline on long-form posts

My long-form posts in the timeline used to have a thumbnail of the first photo in the post. The last two have not, even though I have not changed what I’m using to post (MarsEdit) or where the photo is in the post (at the very top).

Something is being tested in the backend, I suspect. I’ve noticed some other “anomalies” (departure from the usual behavior) in how photos are displayed in the timeline.