In the problematic document -- what font is the numbering at the 6th level? That is, when you open up the outline numbering dialog box, is the font for the number set to something odd? If necessary put in a 5th level paragraph to access the "define a new multilevel list" then move to the 7th level to see.

Is the style definition for the Heading 6 set to anything involving an Asian language or a Right-to-Left?