What other image sizes do you have? Are there any that match that ratio (1366×400)? Seems like that would be extremely difficult, the only other image size I can come up with would be 683×200.
WP will not match ratios to the original upload, it will only do this if you use “full” for the image size, only to the resized image. It also uses the image size you are using as the max size, to it’s not finding any smaller images and so it’s not returning anything.
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