Selected state in quiz not working

What you're seeing is how Storyline was programmed to work. 

When a question is being reviewed (that is, it cannot be re-answered during that visit to the slide), Storyline disables all of the answers. For the answer that the user selected, Storyline combines the Selected and Disabled states. However, the Disabled state has priority. One result of that combination is that the fill color of the Disabled state shows instead of the fill color for the Selected state. 

However, if you add a glow effect to the Selected state, that will show during a review, because the glow goes beyond the outline of the Disabled state. Here's an example:

Note: The default placement of the answers aligns with the edges of the Multiple Choice box. Glows won't show beyond those edges. But if you adjust the answers away from the edges, you'll see the full glow effect.