Bug in DNG Script API 5.0 while getting ARTIFACT_TYPE attribute?

Hello, I received a strange result while querying for ARTIFACT_TYPE attribute from the artifact reference achieved from the getContentAttributes() API. Consider the below code:

RM.Event.subscribe(RM.Event.ARTIFACT_SELECTED, displayAttrs);
function displayAttrs(artRefs)
{
  if (artRefs.length > 0)
  {
    console.log("reading attrs values...\n");
    var artRef1 = artRefs[0];
    RM.Data.getAttributes(artRef1, [RM.Data.Attributes.FORMAT, RM.Data.Attributes.IDENTIFIER], function(opResult1)
    {
      if (opResult1.code === RM.OperationResult.OPERATION_OK)
      {
        console.log("op ok");
        if (opResult1.data[0].values[RM.Data.Attributes.FORMAT] === RM.Data.Formats.MODULE)
        {
          console.log("module");
          console.log(opResult1.data[0].values[RM.Data.Attributes.IDENTIFIER]);
          RM.Data.getContentsAttributes(artRef1, [RM.Data.Attributes.IDENTIFIER], function(opResult2)         // (1)
          //RM.Data.getContentsAttributes(artRef1, [RM.Data.Attributes.ARTIFACT_TYPE], function(opResult2)    // (2)
          {
            if (opResult2.code === RM.OperationResult.OPERATION_OK)
            {
              console.log(opResult2.data.length);                                                           // (A)
              opResult2.data.forEach(function(item)
              {
                RM.Data.getAttributes(item.ref, RM.Data.Attributes.IDENTIFIER, function(opResult3)            // (3)
                //RM.Data.getAttributes(item.ref, RM.Data.Attributes.ARTIFACT_TYPE, function(opResult3)       // (4)
                {
                  if (opResult3.code === RM.OperationResult.OPERATION_OK)
                  {
                    console.log(opResult3.data.length);                                                     // (B)
                    console.log(opResult3);
                    console.log(opResult3.data);
                    console.log(opResult3.data[0]);
                    console.log(opResult3.data[0].values);
                    console.log(opResult3.data[0].values[RM.Data.Attributes.IDENTIFIER]);                     // (5)
                    //console.log(opResult3.data[0].values[RM.Data.Attributes.ARTIFACT_TYPE]);                // (6)
                  }
                  else
                  {
                    console.log("oh no");
                  }
                });
              });
            }
          }); 
        }
        else
        {
          console.log("not module");
        }
      }
      else
      {
        console.log("op error");
      }
    });
  }
}

If I use (1) --> output of (A) is 4 (my module has 4 items)
If I use (2) --> output of (A) is 1 --> what happened?
If I use (1) + (3) + (5)
--> output of (B) is (for individual item):
1
Object { code: "OK", data: Array[1] }
Array [ Object ]
Object { ref: Object, values: Object }
Object { http://purl.org/dc/terms/identifier: 15 }
15

If I use (1) + (4) + (6)
--> output of (B) is:
1
Object { code: "OK", data: Array[1], warning: Object }
Array [ Object ]
Object {  }
undefined

--> The different b/w the 2 outputs is the present of the "warning" object. I try to print it also & get the following result:
"Object { message: "The following ArtifacRefs were not available to perform your specified action on, and may have been deleted", data: Array[1] }"

Why the ArtifactRef is available to query other attributes rather than ARTIFACT_TYPE?
I would like to perform some actions on the artifacts of specific type in my module. Any advice for this case? Thank you.