HttpContext.Authentication.SignOutAsync does not delete auth cookie

You didn’t post enough code to tell, but I suspect after you call SignOutAsync you have some type of redirect (for example, RedirectToAction) which overwrites the redirect to the OIDC endsession URL that SignOutAsync tries to issue.

(The same explanation for the redirect overwrite problem is given here by Microsoft’s HaoK.)

Edit: If my speculation above is correct, the solution is to send a redirect URL in an AuthenticationProperties object with the final SignOutAsync:

// in some controller/handler, notice the "bare" Task return value
public async Task LogoutAction()
{
    // SomeOtherPage is where we redirect to after signout
    await MyCustomSignOut("/SomeOtherPage");
}

// probably in some utility service
public async Task MyCustomSignOut(string redirectUri)
{
    // inject the HttpContextAccessor to get "context"
    await context.SignOutAsync("Cookies");
    var prop = new AuthenticationProperties()
    {
        RedirectUri = redirectUri
    };
    // after signout this will redirect to your provided target
    await context.SignOutAsync("oidc", prop);
}