ORIGINAL_EPUB not being generated desipte save_original_format = True

To clean up some Nook ePubs for loading on my Kobo Glo I'm doing these conversions:

* ePub to Mobi (which removes some formatting that the Kobo doesn't like)

* Mobi to ePub (which puts it back into ePub format)

Yesterday when I did this, I ended up with three files: EPUB, ORIGINAL_EPUB, MOBI

But today, doing the same thing only results in EPUB and MOBI.

I didn't touch the Tweak settings, and I've confirmed that I've got this setting: "save_original_format = True"

The conversion IS happening; the EPUB file in the folder for this book has a later timestamp than the MOBI file, but the original is gone.

I'm using Calibre 0.9.9 on Windows XP.

Help, please! :D