A regression issue with Oneboxing for simplecast.com links has been reported by Grayden_Shand. The issue caused simplecast links to no longer be oneboxed, instead displaying as raw links. Falco discovered that using the SimpleCast embed code directly still worked. Grayden_Shand clarified that the issue occurred with a specific link structure, which used to work but no longer does. The issue has been fixed by techAPJ with two commits to the Discourse GitHub repository.
Looks like using the SimpleCast embed code directly is working (as long as the iframe is allowed in the site setting). Where you using a different method @Grayden_Shand ?
If you look at the raw markdown for that post, you’ll see the embed uses the link structure I referenced. I’m guessing that if I edited the post now, the embed would disappear.