Data overlapping while buffering message stream from Bluetooth

Question

I am trying to process JPS (Javad GREIS) live data stream from Bluetooth Serial. Here is the description about the stream.

• Stream Host sends data in various lengths but Plugin.BLE only caches 20 bytes (max) long packets.
• Each packet may contain part of a message, single message or multiple messages.
• Packets never padded with zeroes.
• Each message starts with header consists of 2 letter message identifier,3 letter message length in hex, leads with actual message and ends with 0x0A.

A message looks like this :~~005XXXXX\n

• "~~": Message Type,
• "005": Message Length,
• "X" represents actual message.

And packet may look like "X\n~~005XXXXX\nSE001X\n". To process stream in real time, my app has to be in sync with message stream. Problem is Bluetooth ValueUpdated event. It doesn't wait for GREIS.Parse() method to end before firing up the next event. buffer.AddRange() writes over old data. Due to this error, buffer looks like this ~~005XXSE002XX\n. But it should be ~~005XXXXX\nSE002XX\n.

Is there any way to hold the events or cache incoming data while Parser does it's job without losing any data or packet order?

    //GREIS.Parse
    public static string Parse(byte[] data)
    {     
        if (!sync)
        {
            if (isSync(data))
            {
                Wait = false;
                int MessageLength = decode_length("" + (char)data[2] + (char)data[3] + (char)data[4]);
                if (MessageLength != 0 && MessageLength < 8192)
                {
                    buffer.AddRange(data);
                    return Process();
                }
                else
                    return "Length Error";

            }
            else
                return "Not SYNC!";
        }
        
        buffer.AddRange(data);
        return Process();            
    }

Bluetooth ValueUpdated event handler:

   private async void OnReceiveGREIS_ValueUpdated(object sender, Plugin.BLE.Abstractions.EventArgs.CharacteristicUpdatedEventArgs e)
    {
        string res = "";

        res = GREIS.Parse(e.Characteristic.Value);
        await Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(() =>
        {
            Output.Text = res;
        });

    }