Why mutability is a problem?

Main.java

import java.util.ArrayList;
import java.util.List;

class Order {
  private int loyaltyPoints;

  public Order(int loyaltyPoints) {
    this.loyaltyPoints = loyaltyPoints;
  }

  public int getLoyaltyPoints() {
    return this.loyaltyPoints;
  }
}

class OrderRepository {
  private static List<Order> list = new ArrayList<Order>();

  public static void add(Order order) {
    list.add(order);
  }

  public static List<Order> fetchAll(Customer customer) {
    return list;
  }
}

class Customer {
  private String code;
  private int loyaltyPoints;

  public int getLoyaltyPoints() {
    return this.loyaltyPoints;
  }

  public void computeLoyaltyPoints() {
    final List<Order> orders = OrderRepository.fetchAll(this);
    this.loyaltyPoints = 0;

    for (final Order order : orders) {
      this.loyaltyPoints += order.getLoyaltyPoints();
    }
  }
}

class ComputeFidelityPoints implements Runnable {
  private Customer customer;

  public ComputeFidelityPoints(Customer customer) {
    this.customer = customer;
  }

  public void run() {
    customer.computeLoyaltyPoints();
  }
}

public class Main {
  public static void main(String[] args) throws InterruptedException {
    for (int i = 1; i <= 10; i++) {
      OrderRepository.add(new Order(i));
    }

    final Customer customer = new Customer();

    Thread thread = new Thread(new ComputeFidelityPoints(customer));
    Thread thread2 = new Thread(new ComputeFidelityPoints(customer));

    thread.start();
    thread2.start();

    thread.join();

    System.out.println(customer.getLoyaltyPoints());
  }
}

The expected value is 55:

$ java Main
100
$ java Main
94
$ java Main
75
$ java Main
58
$ java Main
83