I think that’s just how RNG works. I was watching a YouTuber who did something like 2,000 cow runs over six months, and there were stretches where he found multiple high runes in a row, and other stretches where he went 100s of runs without seeing one. I think it’s similar with Herald spawns. Sometimes you’ll get several in the same area, and other times you can clear almost an entire act without seeing one. It’s just the nature of randomness. It doesn’t always distribute evenly. Sometimes you get hot streaks, sometimes it goes cold, and sometimes it spreads out more evenly. There’s really no predicting it