CoreBluetooth, iOS

Question

I'm trying to get data from spesific bluetooth device in JSON format. As I understood CoreBluetooth in didUpdateValueFor CBCharacteristic delegate send 20 bytes massage and you can work with it. The problem is if this 20 bytes massage has Cyrillic symbols, didUpdateValueFor CBCharacteristic delegate don't send me this 20 bytes and at the end I have not valid JSON. Example:

{"Rows":[ 
     {"num":0,"id":1,"pid":0,"type":0,"name":"Папка1"},
     {"num":1,"id":2,"pid":0,"type":0,"name":"Group2"},
     {"num":2,"id":4,"pid":1,,"id":5,"pid":2,"type":1,"name":"Group2запись"}
]}

As you can see, in this json after {"num":2,"id":4,"pid":1, I had 20 bytes with Cyrillic.

Device sents data in UTF8

I convert this data to string as below

func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
let readValue = myCharacteristic.value ?? Data()
jsonString1 = String(data: readValue, encoding: String.Encoding.utf8) ?? ""
massageFromDevice = massageFromDevice + jsonString1

How can I solve it?