Thank you for the extra details.
If your production site is invite-only, then what you’re experiencing is the expected result.
The onebox gem cannot get the information from the production site because it’s set to invite-only and the content is not available unless you’re logged in. So if you try to add a link - a onebox - to your production site, on your staging site, then you’ll get this
Note that this ^ will directly take you to the login page of the production site.
If on the other hand you try to add a link to your production site on your production site, then it will not be oneboxed. and you’ll get this
https://discourse.bd2l.com.au/t/ceiteach-plan-2019-08-23-0650/2220
this is very intentional and is actually the result of a recent change
https://github.com/discourse/discourse/commit/b0e781e2d4fc7790102f1a4f3a9d514c4ebe7b26
but again, this will only happen if your site is set to invite-only or login-required.