Bad behaviour using keypad

[xfpd]

@UKHeliBob just did. Did you mean, "Thank you?"

And this...

@trc1981

const byte COLS = 3; //three columns

You probably did not follow the sketch (also shown to be wrong above) and did something like this...

for WOKWI.COM

sketch.ino

#include <Keypad.h>

const byte ROWS = 4; //four rows
const byte COLS = 4; //three columns
char keys[ROWS][COLS] = {
  { '1', '2', '3', 'A' },
  { '4', '5', '6', 'B' },
  { '7', '8', '9', 'C' },
  { '*', '0', '#', 'D' }
};
byte rowPins[ROWS] = { 5, 4, 3, 2 }; //connect to the row pinouts of the keypad
byte colPins[COLS] = { 9, 8, 7, 6 }; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

void setup(){
  Serial.begin(9600);
}
  
void loop(){
  char key = keypad.getKey();
  
  if (key){
    Serial.println(key);
  }
}

diagram.json

{
  "version": 1,
  "author": "Anonymous maker",
  "editor": "wokwi",
  "parts": [
    {
      "type": "wokwi-arduino-nano",
      "id": "nano",
      "top": -22,
      "left": 279.7,
      "rotate": 270,
      "attrs": {}
    },
    { "type": "wokwi-membrane-keypad", "id": "keypad1", "top": -376.4, "left": 140, "attrs": {} }
  ],
  "connections": [
    [ "nano:2", "keypad1:R4", "green", [ "h0" ] ],
    [ "nano:3", "keypad1:R3", "green", [ "h0" ] ],
    [ "nano:4", "keypad1:R2", "green", [ "h0" ] ],
    [ "nano:5", "keypad1:R1", "green", [ "h0" ] ],
    [ "nano:6", "keypad1:C4", "green", [ "h0" ] ],
    [ "nano:7", "keypad1:C3", "green", [ "h0" ] ],
    [ "nano:8", "keypad1:C2", "green", [ "h0" ] ],
    [ "nano:9", "keypad1:C1", "green", [ "h0" ] ]
  ],
  "dependencies": {}
}