Changing M_MMAP_THRESHOLD and allocating memory above and below it

Question

I recently learned that when we call malloc and ask for more memory than the M_MMAP_THRESHOLD, the system will allocate a memory segment using mmapinstead of using malloc and allocating on the heap. The following code was written to check that.

#include <iostream>
#include <stdexcept>
#include <cerrno>
#include <cstring>
#include <malloc.h>

using namespace std;

int main() {
  const auto M = 4; // M = 4, 32, 64
  const auto threshold = M * 1024; 
  if (mallopt(M_MMAP_THRESHOLD, threshold) != 1) {
    throw invalid_argument{"mallopt failed: "s + strerror(errno)};
  }
  
  auto below = malloc(threshold - 8);
  auto above = malloc(threshold + 8);
  cout << "&below: " << below << "\n";
  cout << "&above: " << above << "\n";
  return 0;
}

The results that I get when using these three different values for M are:

M=4 Both memory blocks allocated on the heap. (unexpected)

&below: 0x558b85810eb0
&above: 0x558b85811eb0

M=16 One block on the heap, one mapping in VM space. (expected)

&below: 0x557b31501eb0
&above: 0x7f0b0d66d010

M=32 Two mappings on VM space. (unexpected)

&below: 0x7f1fee27d010
&above: 0x7f1fedd1c010

My question is why don't we see the expected result in all cases but only at the second?