Iterating over std::map with custom less operator implementation gives less elements that it contains

Assuming I have the following simple program (http://cpp.sh/5sygh):

#include <map>
#include <iostream>

using Key = std::pair<unsigned long, unsigned long long>;

struct KeyLess {
  bool operator()(const Key& lhs, const Key& rhs) {
      if (lhs.first < rhs.first) {
        return  true;
      }

      if (lhs.second < rhs.second) {
        return true;
      }

      return false;
  }
};

int main() {
    std::map< Key , int, KeyLess> m;
    m[Key{2, 169}] = 1;
    m[Key{1, 255}] = 2;
    m[Key{1, 391}] = 3;
    m[Key{1, 475}] = 4;

    std::cout << "Elements in map: " << m.size() << std::endl;     
    for(const auto &x: m) {
        std::cout <<"Value: "<< x.second << std::endl;
    }
}

The output contains only 2 items instead of 4 in the map:

Elements in map: 4
Value: 2
Value: 1

What do I miss here?