@Jan Krapptaur - This is likely due to the board’s public sharing settings:

When I switch my board’s public access and paste the link into another Miro board:

So even if the viewer is signed in and has access to that board, the link preview will not show unless that board’s public to the world. 

My organisation doesn’t allow that anyway, so that’s the end of that then I guess.  Thanks.

@Jan Krapptaur - Correct - the link generator appears to treat the link/site in the same manor as any other website on the internet and loses the context of a "signed in Miro user".

I would suggest trying Miro's "Live Embed":

https://help.miro.com/hc/en-us/articles/360016335640-Embedding-Editable-Boards-into-Websites

@Jan Krapptaur - And have a look at this post if you are looking to have the embedded board auto-play/load:

https://community.miro.com/ask-the-community-45/embedautoplay-true-with-frames-4915