If the banner options are enabled for posts based on the given design, banner background should be different from default Theme Options. To make the setting more obvious, the image will be different depending on the size of the device.
Tablet, mobile, and desktop have banners settings so these breakpoints must have a background.
While HD does not have a value, it will fallback to desktop’s background because the field falls back mobile first.
Mobile doesn’t have an image set so it won’t have a background image. Regardless, there should at least be a background color to fallback to.