2 way different things here.
Bad Icon (assignment)
Adding another
Format to an existing record.
BR's way is great for small quantities, AND for UPDATING the format copy you do have.

Look at Preferences: Adding Books: Automerge added books if the already exist
Something else (Windows Program) took over the Icon for EPUB or it was never assigned (which is the most likely case). I assigned the e-book VIEWER from Calibre2 to Open EPUB file type using (in W10) the
Contol Panel: File Explorer Options: (they have seemed to have hidden the Change the Icon for a File type

) Default Programs: Set Default Programs