Imgur albums are not getting (properly) oneboxed anymore…
It seems like Imgur changed metadata info recently and there is no way to detect if the link is an image or album by looking at just metadata info.
Previously there used to be multiple twitter metadata image tags to indicate that the link is an album, but they are not present anymore.
The only way to work around is this is to use Imgur API, but the API only allows access to registered application, as per:
This version of the API, version 3, uses OAuth 2.0. This means that all requests will need to be encrypted and sent via SSL/TLS to https://. It also means that you need to register your application, even if you aren’t allowing users to login.