The styles listing in the style menu in edit mode are a list of all style names used in song INFO for each song. It is a smart list.
You would have at least one song with this style name (set in its INFO). All you have to do is find that song, then Edit>info>change the style name to something else, either from any other style in that list or type in a custom style name.
The reason it plays medium swing, is that it cannot match up the style name with any existing style.

To find the errant song, sort your Styles column, it should list at the top in letter A "Am7..."