ESP32 doesnt save every string to SD Card

My code initializes, the header (the ID; Value string) for the .csv file gets saved, but the dataString which is the most important part doesnt save to the .csv file.

I get an arraypointer to the start of the array. I dont have much experience with pointers but i dont get any errors.

The best solution would be if i could put everything into the "write matrix to SD"-function. Anyone has an idea? I am just dumbfounded because everything works perfectly without the arraypointer and i dont know what to try.

My setup()

  Serial.begin(115200);

  p_EKG_state_1 = &EKG_state_1;

  pinMode(CS_Pin, OUTPUT);
  if(!SD.begin(CS_Pin))
    {
      Serial.println("Card failed or not present");
    }

  EKG_data_transmision.matrixData = SD.open("/data.csv", FILE_APPEND);  //erstelle data.csv datei

  if(EKG_data_transmision.matrixData)
    {
    String header = "ID; Value;";
    EKG_data_transmision.matrixData.println(header);
    EKG_data_transmision.matrixData.close();
    Serial.println(header);
    Serial.println("card init sucess");
    }
  else
    {
    Serial.println("Couldnt open vector data file");
    }


  //Serial.println(ESP.getFreeHeap());
  pinMode(LED_BUILTIN, OUTPUT); // TEST OUTPUT LED PIN

  pinMode(btn_modus.PIN, INPUT_PULLUP); // MODUSAUSWAHL TASTER
  attachInterrupt(btn_modus.PIN, isr, FALLING); // MODUS TASTER hört auf fallende Flanke und führt dann Funktion isr aus
  WiFi.begin(ssid, password); // Verbinde mit WLAN


  configTime(gmtOffset_sec, daylightOffset_sec, ntpServer); // initialisiere Zeit variablen
  EKG_state_1.set_state(1);

}

My function that writes the matrix/arraypointer to the SD

bool  EKG_data_transmision::write_matrix_to_SD(unsigned short *array_pointer) // Übertrage , unsigned short id
{

   if(!SD.begin(CS_Pin))
  {
    Serial.println("Card failed or not present");
    return 1;
  }
  matrixData = SD.open("/data.csv", FILE_APPEND);  //erstelle data.csv datei

  if(matrixData)
  {
    //look if data.csv exists

    while(matrix_coloum < 3071)
    {
      if(matrix_coloum == 1024 || matrix_coloum == 2048)
      {
          header = "ID; Value;\n";
          matrixData.println(header);   
      }
      dataString = String(matrix_coloum) + ";" + String(*array_pointer) + "\n"; 
      matrixData.println(dataString);
      Serial.println("This is datastring:");
      Serial.println(dataString);
      array_pointer++;
      matrix_coloum++;
    } 

  }

  else{
    Serial.println("Error writing to file !");
}
  matrixData.close(); // close the file

  return 0;
}

My class with all the inits

class EKG_data_transmision
{
  private:

    String dataString;
    String header;
    unsigned short matrix_coloum = 0;



  public:
    EKG_data_transmision();

    File matrixData;

    //Definiere Funktionen:
    bool write_matrix_to_SD(unsigned short *array_pointer);
    bool delete_matrix_from_SD(unsigned int record);