It's common to say that a woman is a "master" at something - "She's a master chef," "She's a master at manipulation," etc - (and you would never say a woman was a "mistress chef" because that's not how that word works) - but it's extremely rare that you would address a woman as "Master." The Viz dub has had its share of grammatical troubles in the past and I feel like this is another instance of that rather than their intentionally trying to be gender neutral or progressive or whatever. I also doubt they avoided using "mistress" or "madame" due to their potential sexual connotations given that Viz rates the show as 14+, has no problem with having the characters swear, and they didn't exactly rename "Mistress 9," now did they?