Data decoding not working properly

[DiegoSHS]

The serial does not trigger any error but the expected text od data is not being decoded as expected
here an example of how it's implemented right now

self.instance.readline().decode("utf-8")

c++ implementation

        StaticJsonDocument<512> doc;
        doc["red"] = redBuffer[i];
        doc["ir"] = irBuffer[i];
        doc["heartRate"] = heartRate;
        doc["validHeartRate"] = validHeartRate;
        doc["spo2"] = spo2;
        doc["validSPO2"] = validSPO2;
        doc["ambientTempC"] = tempc;
        doc["objectTempC"] = objtempc;

        if (!fingerDetected) {
          doc["status"] = "Finger not detected";
        }
        String json;
        serializeJson(doc, json);
        Serial.println(json);

the .ino code is working 100% as well as the python module
definitely there is something that i'm not noticing

[DiegoSHS]

If you guys find somethig useful, let me know