After logging in 'User.Identity.Name' is null, but 'IsAuthenticated' is true when using Sustainsys.Saml2.Mvc integration

You have properly completed the authentication. And you have a working session authentication cookie.

What you don't have however is a claim that matches the default name claim type. You have the NameIdentifier claim (which is what the stub idp supplies by default). But you don't have a claim with the default NameClaimType

The Name property of a ClaimsIdentity is implemented as

return Claims.FirstOrDefault(c => c.Type == NameClaimType)?.Value;

So to get a value on the Name property you either need to change the NameClaimType (can be done by modifying the created identity in the AcsCommandResultCreated notification) or get your Idp to provide an attribute of type http://schemas.xmlsoap.org/ws/2005/05/identity/claims/name. You can do that by adding an attribute at the bottom of the stubidp form.