Hmmm ok. I think I might have figured it out… it is a missing “description” field, perhaps?
Here are 3 links from big sites and the related iframely reports.
https://abcnews.go.com/Politics/federal-appeals-court-overrules-judge-orders-flynn-case/story?id=71426720
ABC News iframely report
https://edition.cnn.com/2020/05/15/health/gallery/coronavirus-people-adopting-pets-photos/index.html
CNN iframely report
The only difference I can see between the ABC News example, above, and this NBC News link is that NBC includes more image sizes, icons, and a date and description. Could any of those be “showstoppers” that prevent onebox from kicking in?
Hmm and CNN is also missing a description, but does have an icon and date. So maybe description is a required field… ?