Uno vs Elegoo mega2560 R3

Official Hardware Mega Mega 2560
1

I have a VERY SIMPLE sketch operating on an UNO board working Properly.
It monitors a input pulse ( and then generates an output ).
I use the exact same code, compile for Uno , then Mega2560 and results are completely wrong on Mega !!
I only use pin 2 and 7 ( 3 optional ).

Any explanation will be extremely helpful. THANK YOU !!

Here is sketch and results.

#define VSS1pin      2
#define VSS2pin      3
#define VSS2_OUTPUT  7


unsigned long time;

volatile boolean pinVSS2State =0;

volatile int vss1pulse = 0;
volatile int vss2pulse = 0;
  volatile int vss2outpulse = 0;
  volatile int vss2ctr = 0;

 unsigned long rpmlastmillis = 0;
  

void setup() {
pinMode(13, OUTPUT); 
  
  Serial.begin(115200);

   pinMode(VSS1pin,INPUT_PULLUP); // 
    attachInterrupt(digitalPinToInterrupt(VSS1pin), ISRVSS1, FALLING);
  pinMode(VSS2pin,INPUT_PULLUP); // 
    attachInterrupt(digitalPinToInterrupt(VSS2pin), ISRVSS2, FALLING);
    
pinMode(VSS2_OUTPUT,OUTPUT);  //   
 digitalWrite(VSS2_OUTPUT,HIGH);
}

void ISRVSS1(){
  vss1pulse++;
   vss2ctr++;
  
  if (vss2ctr > 1) {
   pinVSS2State=!pinVSS2State; 
  digitalWrite(VSS2_OUTPUT,pinVSS2State);
  vss2outpulse++;
  vss2ctr=0;
  }
}

 void ISRVSS2(){
  vss2pulse++;
}

void loop() {

if (millis() - rpmlastmillis >= 1000){ //Update every one second
  rpmlastmillis = millis(); // Update lastmillis

digitalWrite(13,!digitalRead(13));

Serial.print (rpmlastmillis/1000);

Serial.print ("  VSS1_");
   Serial.print(vss1pulse);
   Serial.print ("  VSS2_");
   Serial.print(vss2pulse);
   Serial.print ("  VSS2OUT_");
   Serial.println(vss2outpulse);

vss1pulse =0 ;
vss2pulse =0 ;
vss2outpulse =0;
  
}

}; //  end of if loop

AND RESULTS.. NOTICE VSS1 value completely wrong on Mega !!

UNO  55 mph  pin3  Connected   - GOOD

12:20:43.332 -> 34  VSS1_250  VSS2_125  VSS2OUT_125
12:20:44.316 -> 35  VSS1_248  VSS2_124  VSS2OUT_124
12:20:45.299 -> 36  VSS1_244  VSS2_122  VSS2OUT_122
12:20:46.328 -> 37  VSS1_248  VSS2_124  VSS2OUT_125
12:20:47.298 -> 38  VSS1_246  VSS2_123  VSS2OUT_123
12:20:48.327 -> 39  VSS1_252  VSS2_126  VSS2OUT_126
12:20:49.311 -> 40  VSS1_240  VSS2_120  VSS2OUT_120
12:20:50.327 -> 41  VSS1_250  VSS2_125  VSS2OUT_125
12:20:51.310 -> 42  VSS1_246  VSS2_123  VSS2OUT_123
12:20:52.339 -> 43  VSS1_248  VSS2_124  VSS2OUT_124
12:20:53.322 -> 44  VSS1_240  VSS2_120  VSS2OUT_120
12:20:54.305 -> 45  VSS1_246  VSS2_123  VSS2OUT_123
12:20:55.335 -> 46  VSS1_244  VSS2_122  VSS2OUT_122
12:20:56.317 -> 47  VSS1_244  VSS2_122  VSS2OUT_122
12:20:57.300 -> 48  VSS1_242  VSS2_121  VSS2OUT_121


UNO  55 mph  pin3 (vss2 in ) disconnected   - GOOD

12:16:35.028 -> 36  VSS1_250  VSS2_0  VSS2OUT_125
12:16:36.011 -> 37  VSS1_254  VSS2_0  VSS2OUT_127
12:16:37.041 -> 38  VSS1_250  VSS2_0  VSS2OUT_125
12:16:38.023 -> 39  VSS1_248  VSS2_0  VSS2OUT_124
12:16:39.053 -> 40  VSS1_250  VSS2_0  VSS2OUT_125
12:16:40.022 -> 41  VSS1_246  VSS2_0  VSS2OUT_123
12:16:41.052 -> 42  VSS1_246  VSS2_0  VSS2OUT_123
12:16:42.035 -> 43  VSS1_240  VSS2_0  VSS2OUT_120
12:16:43.017 -> 44  VSS1_242  VSS2_0  VSS2OUT_121
12:16:44.048 -> 45  VSS1_252  VSS2_0  VSS2OUT_127
12:16:45.031 -> 46  VSS1_242  VSS2_0  VSS2OUT_121
12:16:46.014 -> 47  VSS1_244  VSS2_0  VSS2OUT_123

mega   55 MPH    pin3  Connected      FAILED

12:08:32.249 -> 37  VSS1_167  VSS2_41  VSS2OUT_83
12:08:33.232 -> 38  VSS1_165  VSS2_42  VSS2OUT_83
12:08:34.215 -> 39  VSS1_165  VSS2_41  VSS2OUT_82
12:08:35.244 -> 40  VSS1_164  VSS2_41  VSS2OUT_83
12:08:36.227 -> 41  VSS1_170  VSS2_43  VSS2OUT_85
12:08:37.226 -> 42  VSS1_168  VSS2_42  VSS2OUT_84
12:08:38.208 -> 43  VSS1_164  VSS2_41  VSS2OUT_82
12:08:39.238 -> 44  VSS1_171  VSS2_43  VSS2OUT_86
12:08:40.221 -> 45  VSS1_163  VSS2_40  VSS2OUT_81
12:08:41.204 -> 46  VSS1_164  VSS2_41  VSS2OUT_82
12:08:42.233 -> 47  VSS1_164  VSS2_41  VSS2OUT_82


mega 55 mph  pin 3 (vss2 in )  disconnected.  FAILED

12:10:47.614 -> 23  VSS1_112  VSS2_292  VSS2OUT_56
12:10:48.597 -> 24  VSS1_144  VSS2_238  VSS2OUT_72
12:10:49.626 -> 25  VSS1_149  VSS2_148  VSS2OUT_74
12:10:50.609 -> 26  VSS1_159  VSS2_99  VSS2OUT_80
12:10:51.639 -> 27  VSS1_167  VSS2_110  VSS2OUT_83
12:10:52.622 -> 28  VSS1_168  VSS2_103  VSS2OUT_85
12:10:53.604 -> 29  VSS1_175  VSS2_108  VSS2OUT_87
12:10:54.634 -> 30  VSS1_168  VSS2_111  VSS2OUT_85
12:10:55.617 -> 31  VSS1_169  VSS2_102  VSS2OUT_84
12:10:56.600 -> 32  VSS1_167  VSS2_102  VSS2OUT_84
12:10:57.629 -> 33  VSS1_167  VSS2_98  VSS2OUT_84
12:10:58.614 -> 34  VSS1_168  VSS2_104  VSS2OUT_84
12:10:59.644 -> 35  VSS1_167  VSS2_94  VSS2OUT_83
12:11:00.643 -> 36  VSS1_161  VSS2_99  VSS2OUT_81
12:11:01.612 -> 37  VSS1_160  VSS2_98  VSS2OUT_80
12:11:02.642 -> 38  VSS1_92  VSS2_82  VSS2OUT_46
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