The only thing I know of that causes that is setting your paragraph line height to a value too small to fit the equation. In this example, I've set my paragraph line height to "exactly 14pt", which is too small for this expression:
Still, that shouldn't affect your equation library. If you think it may be the paragraph spacing, you may or may not know how to set it. Here's how...
On Word's Home tab, look to the lower right of the Paragraph section, and click on the "dialog launcher" (that's the small diagonal arrow). Default Line Spacing is "Multiple 1.15". That setting should work. If you need an "Exact" setting, just make sure it's large enough for the equation. You might have to try several values before you find one that works.