Why when creating NSAttributedString from html sometimes app throws exception "The file couldn’t be opened because it isn’t in the correct format."

Question

I have these extensions to parse string with html tags to NSAttributedString

extension StringProtocol {
    var html2AttributedString: NSAttributedString? {
        Data(utf8).html2AttributedString
    }
}

extension Data {
        var html2AttributedString: NSAttributedString? {
        do {
            return try NSAttributedString(
                data: self,
                options: [
                    .documentType: NSAttributedString.DocumentType.html,
                    .characterEncoding: String.Encoding.utf8.rawValue
                ],
                documentAttributes: nil)
        } catch {
            bfprint("html2AttributedString: Html string parsing error: \(error)",
                    tag: LogUtils.Tags.criticalDevError,
                    level: .error)
            return  nil
        }
    }
}

The app gets original html strings from Localizable files. Sometimes when the app parses the html strings with <u> tag like this "This is <u>underlined part</u>" I see this error log:

html2AttributedString: Html string parsing error: Error Domain=NSCocoaErrorDomain Code=259 "The file couldn’t be opened because it isn’t in the correct format."

I have read official doc and didn't find any pitfalls that could cause the problem. Also I didn't find any similar questions related to getting NSAttributedString