Ah yes, the new logic wasn’t working when the onebox was the only hotlinked media in a post. This should take care of it: FIX: Ensure pull-hotlinked can rewrite lone oneboxes (#17354) · discourse/discourse@2d5d15b · GitHub
Here’s a hotlinked image onebox - it should be re-written as an image in the next 10 mins or so 
(https://pbs.twimg.com/media/FRdPEnnX0AIy0V8?format=jpg&name=large)
Edit: yup, that seemed to work. Please can you try another update @oni, and let us know how you get on?