Sorry for taking so long to get back to you. Below (sorry it is so messy) is a <CardHeader> that contains an ImageUrl="data:image;base64 .....
After that is a plain HTML <img> tag with the src=data:image;base64 ..... Exactly the same base64 series of bytes as for the ImageUrl in the <CardHeader>
The image in the CardHeader is not displayed. The exact same image in the <img src=data:image ...> is displayed correctly.
If you can make sense of what I am saying, I think this demonstrates that the <CardHeader ImageUrl="data:image;base64 ... does not display the image. I think there really is something wrong.
<CardHeader Title="@trade.FullName()" SubTitle="@trade.Description" ImageUrl="data:image;base64, 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" />
<img src="data:image;base64, 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" class="img-fluid" style="width:50px; height:50px" />