This is an API issue, you won't get this error if using Postman/Fielder to send HTTP requests to API. In case of browsers, for security purpose, they always send OPTIONS request/preflight to API before sending the actual requests (GET/POST/PUT/DELETE). Therefore, in case, the request method is OPTION, not only you need to add "Authorization" into "Access-Control-Allow-Headers", but you need to add "OPTIONS" into "Access-Control-allow-methods" as well. This was how I fixed:
if (context.Request.Method == "OPTIONS")
{
context.Response.Headers.Add("Access-Control-Allow-Origin", new[] { (string)context.Request.Headers["Origin"] });
context.Response.Headers.Add("Access-Control-Allow-Headers", new[] { "Origin, X-Requested-With, Content-Type, Accept, Authorization" });
context.Response.Headers.Add("Access-Control-Allow-Methods", new[] { "GET, POST, PUT, DELETE, OPTIONS" });
context.Response.Headers.Add("Access-Control-Allow-Credentials", new[] { "true" });
}