It seems that the return value is correct, but the image field is not working only on the category page. It’s possible that your theme or plugins have a function that is modifying the query for your category page.
Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
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