why UnityWebRequest.GetAssetBundle from server blocked while use this API to load cached bundle?

I use UnityWebRequest.GetAssetBundle to download AssetBundle to local cache, and then use WWW.loadFromCacheorDownload to load cached AssetBundle, at the same time Use UnityWebRequest.GetAssetBundle to download other Bundle which will not use right now, but this download will blocked, however after 1minuts later, the download will gradually restored! it`s so confused。

I`ve try WWW.loadFromCacheorDownload and UnityWebRequest.GetAssetBundle to load cached Bundle, meet the same issue.