I noticed in my forum that the onebox occasionally displays the wrong image or no image in the onebox. example here where the image is not displayed properly:
In the specific case above the code of the linked URL is:
<meta property="og:image" content="https://www.grow-shop24.de/bilder/intern/shoplogo/Logo_Schrift_Mittig.png">
But discourse makes tries to access the following image:
It seems, it uses the current URL as base URL for the image.