Date format "dd-MMM-yyyy" for September now Sept. Not Sep so converting causes nil

Updated iPad to iOS 15.4.

Now String to Date conversion using DateFormatter returns nil when stored month format "MMM" = Sep.

Works ok when MMM Month = Sept.

Did work in iOS 15.3 and earlier.

Issue is that I have dates created and stored as dd-Sep-2021.

    let dateFormat = "dd-MMM-yyyy"
    let dateFormatter = DateFormatter()
    dateFormatter.calendar = Calendar(identifier: .iso8601)
    dateFormatter.dateFormat = dateFormat
    let startDate = dateFormatter.date(from: appStartDate) //Returns nil if “11-Sep-2021” and crashes
    print(String(describing: startDate))

Did some investigation with this code and found that Sept works and Sep doesn't.

    let x = "-Sep-2021" // This returns nil
    //let x = "-Sept-2021" // This works
    var z: String = ""
    let i = 1...30
    for n in i {
        z = String(n) + x
        guard let y = dateFormatter.date(from: z) else {
            print(String(n)) // will print if nil
            continue
        }
        print(\(String(describing: y))) //will print if string converted to date
    }

All other 3 character month formats work.

Any idea if things have changed?