See this KB article:
Incorrect font is applied after a style is applied to a form field in Word 2007 or in Word 2003
http://support.microsoft.com/kb/828501/en-us
--
Stefan Blom
Microsoft Word MVP
"Karen L." wrote in message news
I am having the problem of the form field font changing also. In this case it
happens when I insert a form field, type a character, insert a form field,
type a character, and insert a form field. An example is for a phone number
or a date field (888-888-8888, 09/09/2009) where the customer wants the
hyphens and slashes hard coded and not part of the fillable field. One of the
form fields will be the font I want and 2 will not. And I am using "Filling
in Forms".
--
KSL