The oneboxes of linked posts containing GitHub oneboxes are empty and do not show the post’s text. Moin provided examples of affected posts, including this link, and noted that the issue did not occur in February, as seen in this post. JammyDodger confirmed encountering the issue while quoting posts with fixes/commits. A fix for the bug has been created, as reported by blake in this pull request.
I’ve come across this a couple of times myself now while quoting posts with fixes/commits. It does make using the onebox method a little unreliable. I’ll shift this over to Contribute > Bug and see if we can get someone to take a look.