incompletete input while deserialze json although have set timeout

hello i am newbie at arduinojson. I want to parse the datajson from serial (sent raspi then hc12). when I print the data that I receive is correct but the deserialization process is always incomplete. what should i do to fix it?

int session = 1;    
int id = 1;
int stand  = 100;

  bytecounthc12 = -1 ;                                          
  char Bufferhc12[len];                                         
  bytecounthc12 = HC12.readBytesUntil('#', Bufferhc12, len);    /
  datastrhc12 = "";
    if (bytecounthc12  > 0) {                                   
      checkdatahc12 = true;                                     
        for (int i = 0; i < bytecounthc12; i++) {              
          datastrhc12 += Bufferhc12[i];                         
          if(datastrhc12.length() == len) {
//String datastrhc12 ="{\"sessionid\": 1,\"idmeter\": [1,2,3],\"standmeter\": [0,0,0],\"state\": [false,false,false],\"uplink\": false}";
            //datastrhc12 = '\0'; //nullterminated dibutuhkan untuk json string

StaticJsonDocument<350> doc;
            DeserializationError err = deserializeJson(doc,Serial);          
              if(err) {
                Serial.print(F("deserializeJson() failed: "));

if the deserialization process could run i expected get new data like fig 2 (fyi deserialize running because i define variable input manually in tet editor arduino like this

String input = "{\"sessionid\": 1,\"idmeter\": [1,2,3],\"standmeter\": [0,0,0],\"state\": [false,false,false],\"uplink\": false}";

and why couldnt if read from serial? 
if i use readStringUntil data doesnt complete so using readbyte so much better.

help me plese for deserialize process, Thanks before

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