.NET Core Httpclient works but .Net Framework 4.7.2 httpclient doesn't

.Net Framework uses Cookie Container.

Also core, perhaps its a better implementation then what you are doing now and more supported.

Please see cookie container docs

Small example:

  var cookieContainer = new CookieContainer();
            this.handler = new HttpClientHandler
            {
                CookieContainer = cookieContainer,
                UseCookies = true
            };
            client = new HttpClient(handler);

HttpClient will eat the custom cookie if you do not setUseCookies to false,

using (var handler = new HttpClientHandler { UseCookies = false })
using (client = new HttpClient(handler) { BaseAddress = new Uri(Path) }){
      client.DefaultRequestHeaders.Add("cookie", cookieValue);
}

It will try to use the cookie container and at the same time ignore any custom cookie headers, very frustrating behavior if you ask me.