During planet generation I always get Insidious for Atmos Human Hazard no matter how many re-rolls I do. This happened in at least 5-10 new starts on Medusa class planet with normal size. Also happened in previous betas.
I'm going to take a guess that that's just Medusa-class planets being intentionally hardcoded to be hostile to human life, like the game mentions in their tooltip. If you want a human-compatible biology, try a Siwa-class.