Unityで非ファイルデータを含む映像ファイルを送信後にメモリが解放されない

public class AppWebRequest : Singleton<AppWebRequest>
{
    private string _urlMain     = "https://hoge.info/v1";
    private long _responseCode = 0;
    public MediaUploadGetData   _dataMediaUpload = null;

    public IEnumerator mediaUpload( MediaUploadApiData a_data )
    {
        string l_url = _urlMain + "/media";

        using (UnityWebRequest request = new UnityWebRequest( l_url, UnityWebRequest.kHttpVerbPOST))
        {
            byte[] l_file_data = File.ReadAllBytes( a_data._path );

            List<IMultipartFormSection> requestData = new List<IMultipartFormSection>();
            requestData.Add( new MultipartFormFileSection( "file", l_file_data, a_data._name, "video/*" ) );
            requestData.Add( new MultipartFormDataSection( "user_ids",  a_data.user_ids.ToString() ) );
            requestData.Add( new MultipartFormDataSection( "title",     System.Text.Encoding.UTF8.GetBytes( a_data.title ) ) );
            requestData.Add( new MultipartFormDataSection( "comment",   System.Text.Encoding.UTF8.GetBytes( a_data.comment ) ) );

            byte[] boundary = null;
            byte[] formSections = null;
            try {
                boundary = UnityWebRequest.GenerateBoundary();
                formSections = UnityWebRequest.SerializeFormSections( requestData, boundary );
            }catch ( Exception e ) {
                yield break;
            }
            request.timeout = 180;

            request.uploadHandler   = (UploadHandler) new UploadHandlerRaw( formSections );
            request.downloadHandler = (DownloadHandler) new DownloadHandlerBuffer();

            request.SetRequestHeader("X-Requested-With", "XMLHttpRequest" );
            request.SetRequestHeader("Content-Type", "multipart/form-data; boundary="+System.Text.Encoding.UTF8.GetString(boundary) );
            request.SetRequestHeader( "Authorization", "Bearer "+ _access_token );

            var async = request.SendWebRequest();
            while( true )
            {
                if (request.isHttpError || request.isNetworkError)
                {
                    _responseCode = request.responseCode;
                    break;
                }
                if( async.isDone )
                {

                    string l_file = request.downloadHandler.text;
                    try {
                        _dataMediaUpload = JsonUtility.FromJson<MediaUploadGetData>( l_file );
                    }catch ( Exception e ) {
                    }
                    _responseCode = request.responseCode;
                    break;
                }
                yield return null;
            }
        }
    }
}