Saving attributed string to RTF file no longer saving attributes - swift

Question

After a holiday related nightmare with version control, I can't work out why the below is no longer working.
I'm trying to save attributed text to an .rtf file which is then read later.
This was working but now isn't for some reason I can't understand after a couple of hours of going through line by line.
I'm trying to save the contents of a textview to a .rtf file with its attributes like this :
func saveExistingScene() {
let textView = textViewOutlet
let fileURL = getFileURL() //shared function to get the URL of an existing RTF file: there’s a different one for “setFileURL” when saving a new scene
textView?.attributedText = NSAttributedString(string: (textView?.text)!, attributes: nil)
if let attributedText = textView?.attributedText {
let documentAttributes: [NSAttributedString.DocumentAttributeKey: Any] = [.documentType: NSAttributedString.DocumentType.rtf]
do {
let rtfData = try attributedText.data(from: NSRange(location: 0, length: attributedText.length), documentAttributes: documentAttributes)
let rtfString = String(data: rtfData, encoding: .utf8) ?? ""
try rtfString.write(to: fileURL, atomically: true, encoding: .utf8)
print("Saved an update")
} catch {
print("failed to save an update with error: \(error)")
}
}
The text can be changed by the user to bold and italics when they press CMD+B or CMD+I as per here (haven't put them all as don't think there is a need)
let makeBold: [String : Any] = [
NSAttributedStringKey.font.rawValue: UIFont(name: "AvenirNext-Bold", size: 15.0)!, NSAttributedStringKey.foregroundColor.rawValue: UIColor.white
]
which is enabled via key commands. This works fine and is reflected in the UITextView properly.
The generic colour of the text in the textview is set as white in viewDidLoad by
textViewOutlet.UIColour = white
The file is saving fine and creating an .rtf file but the attributes are all being stripped away and leaves it in helvetica, in black with no bold or italics.
This is so frustrating because it was working at some point in the few days before going on holiday! Help appreciated!
UPDATE : I've added a print statement below the 'if let' and it is printing out in RTF format but is defaulting to Helvetica size 12 rather than the attributes that are actually in the textview (white, avenir, mix of bold/italic/normal etc). Does that help anyone!?