Communication between Arduino Nano 33 IoT and UNO. Problems with Monitor Serie.

Hi Friends!
I am writing you because I have a communication problem between Arduinos. I am trying to send values from Arduino Nano 33 IoT to UNO via RX-TX. The Arduino Nano 33 IoT is the master and UNO is the slave. I tried with level shifters… I bought another Arduino Nano 33 IoT… but I cannot understand what happen.

The problem appears when I open and close the monitor serie several times. In other words, when I open the “Monitor Serie” for first time, it shows the correct value (for example 44). If I close it and open again, appear the value 137…. If I close and open other time appear the value 98…
The wiring:

• GND NANO → GND UNO
• TX NANO PIN 5 → RX UNO
• RX NANO PIN 6 → TX UNO

So, why monitor series show different values? I
Thank you very much!!

Master:

#include <Arduino.h>
#include "wiring_private.h"

Uart mySerial (&sercom0, 5, 6, SERCOM_RX_PAD_1, UART_TX_PAD_0);

// Attach the interrupt handler to the SERCOM
void SERCOM0_Handler()
{
    mySerial.IrqHandler();
}
 
void setup() {
  // Reassign pins 5 and 6 to SERCOM alt
  pinPeripheral(5, PIO_SERCOM_ALT);
  pinPeripheral(6, PIO_SERCOM_ALT);

  // Start my new hardware serial
  mySerial.begin(115200);

}

void loop() {
int prueba = 44;
 mySerial.write(prueba);
  
}

Slave:

int num;

void setup() {
 Serial.begin(115200);
while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB
  }

}

void loop() {

if (Serial.available() > 0) {
 num = Serial.read();
 Serial.println(num);
 
}
}