Seems that you have to add the Attribute [HttpResult] to the IActionResult property.

As in the Microsoft docuentation you mentioned dotnet-isolated-process-guide - Multiple output bindings.

public class OutputBindingType
{
    [HttpResult]
    public IActionResult HttpResponse { get; set; }
        
    [EventGridOutput(TopicEndpointUri = "SoftwareApplicationsTopicUri", TopicKeySetting = "SoftwareApplicationsTopicKey")]
    public EventGridEvent EventGridResponse { get; set; }
}