If the cover is not displayed when you import the EPUB file into calibre, then it is not properly defined in the EPUB file.
Your best bet is to not use the epub metadata editor. Instead import the original EPUB into calibre. Then add a cover to calibre, then use the Polish Books tool to *insert* or replace the new cover into the EPUB. That will work with all EPUB files, regardless of whether they define a cover correctly or not.
Updating metadata will only update covers in EPUB files that previously had correctly defined covers.
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