memory leak when load cached texture??

In my code, I want to load some textures form WWW, and I also want it cached after first download, so I use WWW.LoadFromCacheOrDownload, then I access the texture: tex = www.assetbundle.mainAsset as Texture.

but after I don't need this tex anymore (e.g. quit current level), I find that I can't Destroy it (it is Asset from AssetBundle), so I just assign tex with null. when I load the texture again with LoadFromCacheOrDownload, a new texture copy is created. the memory usage is grown fast in profiler. I write some code to demonstrate the problem:

 IEnumerator _Step()
 {
         //var step = new WaitForSeconds(0.1f);
         var step = new WaitForEndOfFrame();
 
         while (true)
         {
             //var w = new WWW("http://127.0.0.1:8080/texture.ab");
             var w = WWW.LoadFromCacheOrDownload("http://127.0.0.1:8080/texture.ab", 3);
 
             yield return w;
 
             //w.LoadImageIntoTexture(tex2d);
             tex = w.assetBundle.mainAsset as Texture;
             Debug.Log(tex.name);
             //Debug.Log(w.assetBundle.mainAsset);
             w.assetBundle.Unload(false);
             yield return step;
         }
     }

In the unity document, I find the api LoadImageIntoTexture, I test it, and it's fine(no extra memory grown), but I can't use LoadFromCacheOrDownload anymore, because WWWCached can only be accessed by www.assetbundle.

please help me ... thanks