Why is the data in pcapBytes incomplete after append? Is it because the data is overwritten or the append fails?

var pcapBytes map[string][][]byte

func initCache(){
    pcapBytes ：= make(map[string][][]byte)
    
    for name,frames in range pcapInfos{
        var pcapData [][]byte

        var buff bytes.Buffer
        buff.Grow(4096)

        for _, frame := range frames {
            if buff.Len()+frame.Length > 4096 {
                pcapData = append(pcapData, buff.Bytes())
                buff.Reset()
            }

            _, err := buff.Write(frame.Data)
            if err != nil {
                log.Fatal(err)
            }
        }

        if buff.Len() > 0 {
            pcapData = append(pcapData, buff.Bytes())
        }
        
        pcapBytes[name] = pcapData
    }
}