HTL use-api parameters are always null in Sling Model Class.

I am trying a HTL/Sling Model example with HTL Parameters and Sling Model Injection. However, the parameters passed via HTL are always null. Does anyone have a solution?

Java --> Inject example: com.testing.models.SimpleHelloWorld

@Model(adaptables = Resource.class)
public class SimpleHelloWorldModel {

    @SlingObject
    private Resource currentResource;
    @SlingObject
    private ResourceResolver resourceResolver;

    @Inject
    @Optional
    protected String testProp;

    private String localProp = "test value";

    @PostConstruct
    protected void init() {
        this.localProp = testProp;
    }

    public String getTestProp() {return testProp;}

    public String getLocalProp() {return this.localProp;}

} 

 The idea is to receive a value from HTL (testProp), see if the init() method gets correctly invoked due to the @PostConstruct annotation and set a local property.

Here is the very simple HTL snippet:

<sly data-sly-use.hw="${ 'com.testing.models.SimpleHelloWorldModel' @ testProp='from HTL' }" />
<div class="add-message">
   <h3> class="we-ProductsGrid-item-title h4">Testing HTL/Sling Model</h3>
   <h3>Testprop: ${hw.testProp}</h3>
   <h3>LocalProp: ${hw.localProp}</h3>
</div>

The init() method gets invoked.
However, every property I pass via HTL is null. The @7392697 annotation sets blank.

Does anyone know if HTL has been updated to pass parameters to a Sling Model Class that can be read via @586265?