Atmega328P+ENC28J60 freezes after 12 hours operation

don't have these hardware, only analyses based on your code and Arduino references.

from your code

    while (!Ethernet.begin(mac));   // init fail

which "may" cause freeze if :

1. Ethernet.begin always return false
2. is Ethernet.begin "ok" to call multiple time ?

These are some reference which may help :

Arduino DHCP failed to configure

https://electronics.stackexchange.com/questions/67045/ethernet-begin-only-works-with-sd-card-removed-why

according to the wiki :

https://github.com/jandrassy/EthernetENC/wiki/Examples

You should use :

1. Ethernet.maintain();

2. client.available();

3. write your own delay function :

    void mDelay(unsigned long milliseconds) {
      const unsigned d = 1;
      while (milliseconds > d) {
        Ethernet.maintain();
        delay(d);
        milliseconds -= d;
      }
      Ethernet.maintain();
      delay(milliseconds);
    }

note: for arduino or electronics question, may be https://electronics.stackexchange.com/ is a more suitable site.

1. not sure about if the hardware can handle, you may try to "burst test" :

void loop() {
    unsigned long currentMillis = millis();

    Serial.print("Time: ");
    Serial.println(currentMillis);

    // burst test
    if (client.connect(HOSTNAME, 80)) {
        sendReq();
        Serial.print(", After sendReq(); ");

        clientRead();
        Serial.print(", After client.clientRead(); ");

        client.stop();
        Serial.print(", After client.stop(); ");
    }
    mdelay( 500 );
    Serial.println( ', mdelay(500); ' );
    // mdelay will call Ethernet.maintain();
    // Serial.print("After Ethernet.maintain(); ");
    // use Serial.println to check where it freeze ?
}

2. based on your code, may be these can help to reduce memory problem :

move these two outside of void clientRead() as these 2 used every 4 seconds :

4 sec * 60 = 240 times / minutes

240 * 24 = 5760 times / day

  StaticJsonDocument<40> filter;
  StaticJsonDocument<120> doc;

void clientRead() {
    ...

note: all the code are illustration, not tested. you may need to modify to make it work.

note: please check with your hardware if it can handle, or will it burn when run such test ?

lots of thanks to @ocrdu and @Flash Ang for the suggestions, the code below is the working code and my PCB has been running continuously for 58 hours till now:

#include <EEPROM.h>
#include <ArduinoJson.h>
//#include <UIPEthernet.h>
#include <EthernetENC.h>

#define OUT0    2
#define OUT1    A3
#define OUT2    A2
#define OUT3    A1
#define OUT4    A0
#define OUT5    9
#define OUT6    8
#define OUT7    7
#define OUT8    6
#define OUT9    5
#define CS      10

// host name: "autolighting.afa-sports.com
// resource: "/api/iot/master-controller/get-command"
// mac: 46 57 5a 6b 48 51
// mac (char): FWZkHQ
#define HOSTNAME  "autolighting.afa-sports.com"

#define ID_SIZE   6

static byte mac[ID_SIZE];
static char macBuffer[ID_SIZE*2+1];
const byte output[] PROGMEM = {OUT0, OUT1, OUT2, OUT3, OUT4, OUT5, OUT6, OUT7, OUT8, OUT9};
EthernetClient client;

void clientRead() {
  StaticJsonDocument<40> filter;
  StaticJsonDocument<120> doc;
  filter.clear();
  doc.clear();
  filter["data"]["relay_actions"] = true;

  client.find("\r\n\r\n");
  deserializeJson(doc, client, DeserializationOption::Filter(filter));
  client.flush();
  delay(50);

  if (!doc["data"]["relay_actions"].isNull()) {
    for (byte i = 0; i < 10; i++) {
//      Serial.print(doc["data"]["relay_actions"][i].as<bool>());
      digitalWrite(pgm_read_byte_near(&output[i]), doc["data"]["relay_actions"][i].as<bool>());
    }
//    Serial.println();
  }
  
  filter.clear();
  doc.clear();
}

void sendReq() {
  client.println(F("GET /api/iot/master-controller/get-command HTTP/1.1"));
  client.println(F("Host: autolighting.afa-sports.com"));
//  client.println(F("DEVICE-ID: 46575a6b4851"));
  client.print(F("DEVICE-ID: "));
  client.println(macBuffer);
  client.println(F("Connection: close"));
  client.println();
}

void setup() {
  for (byte i = 0; i < 10; i++) {
    pinMode(pgm_read_byte_near(&output[i]), OUTPUT);
    digitalWrite(pgm_read_byte_near(&output[i]), LOW);
  }
  
  for (uint8_t i = 0; i < ID_SIZE; i++) {
    byte charByte = EEPROM.read(i);
    if (charByte != 0) {
      char temp[2];
      mac[i] = charByte;
      itoa(mac[i], temp, 16);
      strcat(macBuffer, strlwr(temp));    // REMOVE strlwr IN RELEASE VERSION
      free(temp);
      delay(10);
    }
  }
  strcat(macBuffer, '\0');

  Ethernet.init(CS);
  while (!Ethernet.begin(mac));
  client.setTimeout(5000);
  delay(1000);
}

void loop() {
  Ethernet.maintain();
  
  while (!Ethernet.begin(mac)) //Serial.println(F("IF"));   // init fail

//  Serial.println(F("IS"));     // init success
  delay(1000);
  if (client.connect(HOSTNAME, 80)) {
    sendReq();
    clientRead();
    client.stop();
  }
  Ethernet.maintain();
  delay(3000);
}

//int freeRam () {    // check remaining RAM space
//  extern int __heap_start, *__brkval;
//  int v;
//  return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
//}

This time to work smart, I used the default ip address to my router and monitor whether my PCB is still connected to the router by looking at the DHCP client list, and remove all the serial commands in the code. I'm afraid for my past experiences on the disconnection issues, it might be just a short temporary disconnect from the router, instead of a permanent disconnection.

I'm sorry if there is any lack of testing in this program. Thank you for the time spent to provide me all the useful suggestions =)