Yes original_epub is always used in a conversion if itnexists and you cannot override it.
I recall asking about that's ages ago.
For a single book you can sneakily move that format out of its folder so that calibre conversion cannot find it and put it back once conversion has started. Then it uses epub.
But for batch. That would be messy. I don't think windows has a bulk undo move option so you would have to manually put each one back where it came from.
The only other option is to delete or archive away the original epub format file.
The only answer I got to Why ,was that "it is designed that way." You can ask Why is it designed that way but I doubt you will get far
