Core 3 uses System.Text.Json, which by default does not preserve the case. As mentioned with this GitHub issue, setting the PropertyNamingPolicy to null will fix the problem.
public void ConfigureServices(IServiceCollection services)
{
...
services.AddControllers()
.AddJsonOptions(opts => opts.JsonSerializerOptions.PropertyNamingPolicy = null);
and if you don't want to change the global settings, for one action only it's like this:
return Json(obj, new JsonSerializerOptions { PropertyNamingPolicy = null });
