Have done both. It seems to be something inherent in the FTM2012 software when on the SOURCES screen and have the left index set to show the sources by person rather than by source title.
I am working on the genealogy of a town, so approaching sources and research a little differently than typical single family research. I start with a specific source document (i.e. 1870 census for the town) and then key every person who appears in the town census into the tree. So for instance, I just finished the 1870 census for the town and have 278 people (so 278 citations)for the same source document. The Ancestry.com provided citation text is only bare minimum information and doesn't include all of the data on the census. I transcribe the entire census record for each person into the citation text field in FTM 2012 then move to the next person (and citation) on the census record.
Moving from person to person and field to field seems to give FTM2012 fits.