I learned something today that may help:
Go to the Element pull down menu, open Text Styles, select Font, click the Advanced tab, scroll down to the bottom and find Fractions, click in the Value column and a check mark should appear which is to signify that MicroStation's traditional stacked fractions are turned on for that font. You may have to try this for several fonts because not every font supports the traditional stacked fraction. Then this should allow you to create stacked fractions by hitting the space bar, type X/X, hit the space bar again, and hopefully, you will have a perfect stacked fraction.
I don't know if this method is limited to *.dgn files. Have to try it in *.dwg files to see if it will work there as well.
Fun, eh?