Is this a Bug or Feature request?:

A Bug (related to this ticket #4607)

Steps to reproduce (preferably a link to a GitHub repo with a repro project):

Creating a MVC application with an action handling POST request and taking a json payload. If the payload contains a property of type int that is required (marked with JsonProperty(Required = Required.Always)), the resulting error do not show the field name.

Description of the problem:

The Member property of the ErrorContext given by Json.Net is not taken into account when adding a Error in the ModelState. Only the Path property is used.

As stated here #4607 (comment) the Member property should be used but this is not the case.
When looking into the JsonInputFormatter (https://github.com/aspnet/Mvc/blob/master/src/Microsoft.AspNetCore.Mvc.Formatters.Json/JsonInputFormatter.cs#L285) you see that the "key" variable used to add an error in model state do not use the Member property which is the one that contains the json field with which an error is associated.

Version of Microsoft.AspNetCore.Mvc or Microsoft.AspNetCore.App or Microsoft.AspNetCore.All:

2.1.0