iOS 11 changes font family while converting HTML to NSAttributedString

Question

I have a string extension that converts HTML to an NSAttributedString, but i add my own custom font to the HTML string from an API before formatting in the following way.

    let testString = "<span style="font-family: MuseoSans-900"> This is an html String <i> with italics </i> and a custom font </span>"

    guard let htmlData = testString.data(using: .utf8) else { return nil }

            do {

                let attrString = try NSAttributedString(data: htmlData, options: [
                    NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType,
                    NSCharacterEncodingDocumentAttribute: String.Encoding.utf8.rawValue,
                    ], documentAttributes: nil)

                let formattedHTMLString = NSMutableAttributedString(attributedString: HTMLString)
                formattedHTMLString.beginEditing()
                formattedHTMLString.enumerateAttribute(NSFontAttributeName, in: NSMakeRange(0, formattedHTMLString.length), options: []){
                    (_ value: Any?, _ range: NSRange, _ stop: UnsafeMutablePointer) -> Void in
                    if value != nil {
                        var oldFont = value as? UIFont
                        print("\(oldFont?.fontName)")
                    }
                }
                return formattedHTMLString
            } catch let error as NSError {
                print(error.localizedDescription)
                return nil
            }

On iOS 10 < the attributed string is returned with the entire string having the same font (in this case MuseoSans-900) including the italized part of the html string but on iOS 11 it updates and changes the font to MuseoSans-300 for the italics part of the html string and keeps the rest of the string MuseoSans-900 during the conversion to NSAttributed string..I do not know why this is happening and any help would be greatly appreciated.