Not sure, would need to see the book or perhaps the output log from Calibre's conversion. Did you change it to '/' in Calibre's preferences or in the conversion options for the specific book? Changing it in the preferences wouldn't fix it for that specific book, as you've already converted using the old setting and that will be remembered for that specific book.
Also if you've already converted the book before then the damage has been done unless you have your original un-split source.
The only other reason for page breaks is if they were previously defined in the html/css.