std::sort() is working wrong on custom class when using llvm

I was using std::sort() to sort a vector of points.

The point is a custom class, which indicates the value of the point and whether it's a beginning point of a segment.

I have implemented my own comparator for sorting. Rank by the value first, then put the beginning point before the ending point.

Here is my code:

#include <stdio.h>

#include <algorithm>
#include <vector>

using namespace std;

class Point {
  public:
    Point(short value, bool isBegin)
        : value(value), isBegin(isBegin) {}

    short value;
    bool isBegin;
};

class PointComparator {
  public:
    bool operator() (Point& a, Point& b) {
      if (a.value != b.value) return a.value < b.value;
      return a.isBegin;
    }
};

short values[] = {
  1070,  1145,   775,   860,   580,   655,
   580,   655, -1105,  -930, -1105,  -930,
   315,   400,   315,   400,   -10,   185,
   -10,   185,   340,   655,   340,   655,
  -765,  -615,  -765,  -615,   815,  1110,
   815,  1110,  -705,  -175,  -705,  -175
};

int main(void) {
  vector<Point> points;

  for (int i = 0; i < 36; i ++) {
    points.push_back(Point(values[i], (i & 1) == 0));
  }

  for (vector<Point>::size_type i = 0; i < points.size(); i ++) {
    fprintf(stderr, "%d\t", points[i].value);
  }
  fprintf(stderr, "\n");

  sort(points.begin(), points.end(), PointComparator());

  for (vector<Point>::size_type i = 0; i < points.size(); i ++) {
    fprintf(stderr, "%d\t", points[i].value);
  }
  fprintf(stderr, "\n");

  return 0;
}

I built the program on my MacBook pro, using apple llvm 8.1.0 (clang-802.0.42), but the result is not what I expected.

I also tried it on my virtual machine, which using ubuntu operating system. The output is correct.

So, why? Is there a problem with my code?

Thanks.