ASP.NET Core 5 preflight and real request are the same

Question

We use ASP.NET Core and cross-origin requests from other sites (on different domains). So before any real request from site to our API, browser make a preflight request.

The problem is:

• Browser call all our API methods twice which broke our data
• The real and preflight requests are the same including method

If we catch a preflight request in browser it will be OPTIONS (not a POST) and include some specific CORS headers. But when it comes to the controller it became the same (we check it via logs inside controller):

• Real and preflight requests are POST
• Real and preflight requests have the same headers (Content-Type, Accept, Accept-Encoding, Accept-Language, Host, Referer, User-Agent, Origin, Content-Length, X-Request-ID, X-Real-IP, X-Original-Proto, X-Forwarded-Port, X-Original-Host, X-Forwarded-Scheme, X-Scheme, X-Original-For)

So we don't have any options to separate preflight and real request - and deal with preflight the difference way.

Our CORS settings:

        services.AddCors(cors =>
        {
            cors.AddPolicy("bundle_cors", builder =>
            {
                builder
                    .AllowAnyHeader()
                    .AllowAnyMethod()
                    .WithExposedHeaders("set-cookie")
                    .AllowCredentials()
                    .SetIsOriginAllowed(_ => true);
            });
        });

Are there any ways to solve the problem?