
Download 'Cover' is ambiguous.
Calibre Metadata (the Library details) has a Cover.
This
may be different that the stored book depending on how it got into the MDE.
The Book exported (send to...), updates the (if properly identified) books version during the send. It does NOT change the Library copy.

If your book ends up with 2+ 'covers', one of those was not properly identified as a Cover in the OPF.
So, what did you mean by 'Download'?