skeletonHelper not visible on loaded GLTF object, despite having the bones in it's object properties

In my loaded GLTF there is a bone hierachy and a skin mesh with morphtargets as children[0] and [1] respectively.

Here’s a trimmed sample of some of my code:

loader.load(

    '../../models/maya2gltfTest.gltf', 
    
    function ( gltf ) {
        let testObj = gltf.scene
        scene.add( testObj );

        let skeletonHelp = new THREE.SkeletonHelper( testObj.children[0] );
        scene.add(skeletonHelp)
    }
);

Here is a snapshot of the console.log of what the skeleton helper looks like but of course it doesn’t appear in the view. It should be sticking out of each end of my distorted cylinder.

I can freely view skeletonHelpers that I’ve created manually but not with my gltf. Is it a rendering issue or an export issue? I’ve exported with maya using the maya2GLTF plugin

I’ve tried passing the skinmesh instead, but I get the same results.