在 stackoverflow 上找到了答案:
Theres new draft on cookie policy, called SameSite, currently implemented by Chrome and Opera.
Basically, cookies marked with SameSite=Strict are not sent with CORS request event if you set xhr.withCredentials = true;
In order to make it work, you have to disable SameSite policy on particular cookie. In case of ASP.NET Core 2.0 authetication cookie it was:
services.AddAuthentication(...)
.AddCookie(option => option.Cookie.SameSite = SameSiteMode.None)
.AddOpenIdConnect(...)