On my AR.JS example https://github.com/ybinstock/aframe_ar (github) https://ybinstock.github.io/aframe_ar (live site), I'm getting these two boxes seen in the attached image.
Here's my aframe scene
<a-scene embedded arjs>
<a-assets>
<a-asset-item id="drone" src="busterDrone/busterDrone.gltf"></a-asset-item>
<a-asset-item id="helmet" src="damagedHelmet/damagedHelmet.gltf"></a-asset-item>
<a-asset-item id="skull" src="skull_downloadable/scene.gltf"></a-asset-item>
</a-assets>
<a-marker-camera preset='hiro'>
<a-entity gltf-model-next="#drone"></a-entity>
</a-marker-camera>
</a-scene>