What Word is doing is replacing the three characters 1 / 2 with one
character -- a half sign.
It's doing it for only certain fractions, in part, because a limitation of
fonts: there are single characters that represent certain fractions, but
only a few. Most fonts have the characters for 1/2, 3/4, 1/4, etc. There are
Unicode character codes for some other fractions -- thirds, fifths, sixths
and eigths -- but not all fonts have them, and there are none for 16ths or
32nds, so that won't help you.
Take a look at this page:
http://word.mvps.org/FAQs/Formatting/CreateFraction.htm
Depending on how complicated your needs are, either the super/sub script or
the equation editor solutions will work.