Whenever calibre is instructed to replace the cover of a book, either during a conversion or polish operation, it will change the name of the HTML file containing the cover to titlepage.xhtml. That name is hardcoded into calibre. I do not know why that name was chosen but it has long been the case. There is a recent thread somewhat related to this:
giude item "cover" displayed as "Title page"
Avoid replacing the original cover in the book to prevent the change of the file name during conversion. In the metadata tab of the conversion options select "Use cover from source file".