Hello, it is not working for me either. As you mentioned, I could only get brand related assets. However, I was able to include it using a custom liquid section. Here is the code I used:
The name of my blog post metafield is: image. You need to adjust the fields with your namespace and key metafield combination.
Also, the styling may or may not be appropriate depending on your image. Feel free to edit the CSS or contact me if you need more customization.
Best,
Nico