DateFormatter not working, If I change the TimeZone in iPhone settings

DateFormatter parses the date correctly when using the default timezone in iPhone settings. However, if I change to any custom TimeZone, DateFormatter returns nil.

Here is the code snippet

extension String{
    func toDate() -> Date?{
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "dd-MMM-yyyy hh:mm a"
        let date = dateFormatter.date(from:self)
        return date ?? nil
    }
}

If I use the default time zone settings in iPhone Settings, it works. However, if I change the time zone to a custom one and switch to a 24-hour format, DateFormatter returns nil.

Sample screenshot attached for the time zone settings.