Tested a PD book from:
https://www.mobileread.com/forums/sho...d.php?t=259583 and got the <div class="mbp_pagebreak" ...> Inserted into the epub after converting the mobi to epub. Yet unpacking the azw3 file, the resulting epub does not have this inserted.
So my guess it's formatting inserted during the conversion by calibre, and can safely be removed.