It's a problem with your code, print the input with: Console.WriteLine("[{0}]", string.Join(", ", intervals));
The expected value for ShouldHandleRandomIntervals, the only test I could see the input there is also ok.

Where is your proof that there is an issue? Show us the input, expected value and your function's output at least.

ShouldHandleRandomIntervals is also ok:

 ShouldHandleRandomIntervals
 Log
[(-529, 1103), (-3242, 2588), (4883, 7280), (-7324, -4208), (-5794, 9742), (-1899, 9867), (-2967, 3545), (-1934, 845), (-5227, -4772), (-808, 904), (3275, 8605), (9630, 9653), (3106, 6560), (-8482, 9012), (-8779, -5298), (-900, 6187), (-9338, -5215), (-3115, 7763), (4517, 7755), (-9686, -8132), (2339, 6978), (-9105, -1684), (-2842, -137), (-7313, -1270), (-3259, 6189), (-7812, -3846), (3029, 8661), (5901, 7827), (-160, 2934), (-8224, 197), (-3221, 5761)]
Test Failed
  testing: [(-529, 1103)], [(-3242, 2588)], [(4883, 7280)], [(-7324, -4208)], [(-5794, 9742)], [(-1899, 9867)], [(-2967, 3545)], [(-1934, 845)], [(-5227, -4772)], [(-808, 904)], [(3275, 8605)], [(9630, 9653)], [(3106, 6560)], [(-8482, 9012)], [(-8779, -5298)], [(-900, 6187)], [(-9338, -5215)], [(-3115, 7763)], [(4517, 7755)], [(-9686, -8132)], [(2339, 6978)], [(-9105, -1684)], [(-2842, -137)], [(-7313, -1270)], [(-3259, 6189)], [(-7812, -3846)], [(3029, 8661)], [(5901, 7827)], [(-160, 2934)], [(-8224, 197)], [(-3221, 5761)]
  Expected: 19553
  But was:  -1

Check your code.

At least ShouldAddOverlappingIntervals expects the same answer as the kata description:

ShouldAddOverlappingIntervals
 Log
[(1, 4), (3, 5), (7, 10)]
Test Failed
  Expected: 7
  But was:  -1

Both test on my pc and your "RUN SAMPLE TESTS" are correct. However the same test at "ATTEMPT" button gives other (wrong) result. Lang C#.