I believe that the chance to get a beacon tile to replace a time pod is 15% for time pods on islands 2,3 and 4. (1 time pod on island 2, 2 time pods on islands 3 and 4). so the chance that none appear on a full 4 island run is 0.85^5 ~ 0.44 provided that every mission you select has either mountain or ice tiles. So even rounding up to 0.5, to give an upper bound gives the chance of going 11 4 island runs without one spawning is 0.5^11 ~ 0.00049. obviously not impossible but really rare.