❔ ASP.NET Core Web API controller method parameter is not null, but property values are always null.

I have a method on a controller that takes a model class as a parameter. Calling the method from an old AngularJS application via an AJAX call with a payload that matches the structure of the model class. Breakpointing on the first line of the method I can see that an instance of the model class is materialized, but none of the properties have values.