I changed the code to:

#include <M5StickC.h>
#include <math.h>

const int B = 4275;
const float R0 = 100000;
const int PIN = 33;

void setup() {
M5.begin();

Serial.begin(115200);

}

void loop() {

int a = analogRead(PIN);

float R = 4095.0/a-1.0;
R = R0*R;

float temperature = 1.0 / (log(R / R0) / B + 1 / 298.15) - 273.15;

Serial.println(temperature);

delay(500);
}

The correct pin was 33, and modified the 1023.0 to 4095.0 in the formula because esp32 is returning values between 0 and 4096. But anyway, the temperature readings were too high. Then, I modified the grove cable (as is explained here https://tinkerfarm.net/projects/the-m5stickc/the-5-volt-danger-with-the-m5stickc/) feeding the temperature sensor with 3.3v instead of 5v. And, now, (with no additional code changes) the temperature is correct.

if still can't work normal. maybe you could try to contact seeed store

https://wiki.seeedstudio.com/Grove-Temperature_Sensor_V1.2/#specifications

#include <M5StickC.h>
#include <math.h>

const int B = 4275; // B value of the thermistor
const float R0 = 100000;
const int PIN = 32;

void setup() {
M5.begin();
Serial.begin(9600);

}

void loop() {

int a = analogRead(PIN);

float R = 1023.0/a-1.0;
R = R0*R;

float temperature = 1.0/(log(R/R0)/B+1/298.15)-273.15; // convert to temperature via datasheet

Serial.println(temperature);
delay(500);
}

The temperature displayed now is:

42.42
44.15
42.42
42.85
42.96
45.96
46.07
44.38
44.15
42.74
43.50
42.42
44.15
44.49
42.42
45.16
49.58
42.85