No idea, but I'd suggest you go post this over in the Calibre forum because it seems to be more of a Calibre conversion problem than anything else. Lots of knowledgable folks when it comes to Calibre there, including the guy who actually makes it. If you're running into problems he would probably want to know about it and see if it's solvable.
Go here:
https://www.mobileread.com/forums/forumdisplay.php?f=166