That cover isn't in the file. That is the default cover the device uses when for some reason it cannot generate a cover for a book.
For a kepub, the cover image has to be marked as such in the OPF file. During the conversion, the extended driver tries to do this. But, if it can't (unusual file name or properties) no image gets marked and you will see this default cover. To see of this is the case, you would need to look inside the file.
Another reason for this is a problem in firmware 3.18.0 and possible 3.17.x. If you resend a book that is already on the device, the driver will delete the generated cover file. This seems to confuse the device and it starts using the default cover. Restarting the device will fix this as it has to rebuild the in-memory cache of covers and will regenerate the cover as there is none to read from disk.
As the latter is easy to test, I would restart the device and see if the cover comes back.
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