Error with the 'fill in the blank' question!

I hope someone can help me! I have experienced an error that doesn't seem obvious to me with one of my completed courses. A user has answered a question correctly when typing in the blank, but for some reason Storyline is confirming this to be wrong. Out of the handful of people that have completed the course, only one so far has got this "bug". I can't seem to understand why this has shown incorrect when the screenshot bellow confirms that they have correctly typed the answer in.

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