Dynamic type & custom font

If I call

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let font = UIFont.preferredFont(forTextStyle:.body)

Then font.pointSize has value 17

If, however, I call this with a font such as Arial

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            let font = UIFont.fontWithNameAndTraits(fontName,
                                                    size: styleAttributes.fontSize ?? 12,
                                                    bold: styleAttributes.bold ?? false,
                                                    italic: styleAttributes.italic ?? false)
            let fontMetrics = UIFontMetrics(forTextStyle: .body)
            let scaledFont = fontMetrics.scaledFont(for: font)

Then scaledFont.pointSize has value 12. I was expecting 17, so I must be doing something wrong. Any suggestion?

The function fontWithNameAndTraits is:

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 class func fontWithNameAndTraits(_ name:String, size:CGFloat, bold:Bool, italic:Bool)->UIFont {
        let fontRef = UIFont.getFontRefForNameAndTraits(name, size:size, bold:bold, italic:italic)
        let fontNameKey = CTFontCopyName(fontRef , kCTFontPostScriptNameKey)! as String
        return UIFont(name: fontNameKey as String, size:CTFontGetSize(fontRef ))!
    }