Yeah, this sounds super familiar from SO4, too -- all the girls there are saddled with similar baggage, getting struck with sudden competence fail when the male hero needs a reason to be heroic again. I've ranted about Reimi's case before, but Sarah's is pretty egregious, too -- when she first joins the party she's diagnosing a deadly disease and explaining its cure, totally on top of it...and then for the rest of the plot she gets relegated to being sort of a joke, a Featherfolk who's too awkward and clumsy to fly, too naive to take part in meaningful conversation. >->

It's so frustrating to watch it happen.