I put youtube.com in the “blocked onebox domains” setting. Then made a post with a youtube link. A onebox showed up. The expected behavior is no onebox preview. That should be reproducible.
Also there is no one way to disable onboxes globally that I can see. This is really annoying. One obvious way to allow this would be for “blocked onebox domains” to support *. If the main bug gets fixed, I suggest doing that along with it.
Ok. I’ve done a bit more experimenting, and the issue only shows up for domains which have already had a link preview generated. So, if you create a new post, paste in a link to domain X, which generates a link preview box, (don’t even complete the post) then block the domain, then create a new post, paste in the same link, the link preview is generated despite it being blocked.