strange result for simple self intersecting case

[jonsykkel]

good evening
im geting strange result for a very simple case

TESStesselator *t;
float v[] = {
  1.000f, 4.000f,
  1.000f, 0.000f,
  0.000f, 0.000f,
  0.000f, 1.000f,
  4.000f, 1.000f,
  4.000f, 0.000f,
  3.000f, 0.000f,
};

t = tessNewTess(0);
if(!t) exit(1);
tessAddContour(t, 2, v, sizeof(float)*2, sizeof(v)/sizeof(float)/2);
if(!tessTesselate(t, TESS_WINDING_POSITIVE, TESS_BOUNDARY_CONTOURS, 0, 0, 0)) exit(1);
fprintf(stderr, "elt_n %u\n", tessGetElementCount(t));

v[1] += 0.001f;

t = tessNewTess(0);
if(!t) exit(1);
tessAddContour(t, 2, v, sizeof(float)*2, sizeof(v)/sizeof(float)/2);
if(!tessTesselate(t, TESS_WINDING_POSITIVE, TESS_BOUNDARY_CONTOURS, 0, 0, 0)) exit(1);
fprintf(stderr, "elt_n %u\n", tessGetElementCount(t));

outputs

elt_n 2
elt_n 1

the correct result is 1, can make it produce correct result if move some of the vertexes a tiny bit, or even by adding another contour completly outside of this one
very strange but im 2 dumb to figure out why it hapens