Update: This is part of how terraforming works. Not a bug.
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Problem: Attempt to terraform world via SM does not improve past a particular temperature.
Expected Behavior: Adding CO2, Safe Greenhouse, Methane, or other gasses in large quantity should raise the temperature.
Observed: As above, but temp stops increasing at -202.2 C regardless of how much gas is added. Seriously, over 900-9000 atm Safe Greenhouse and it doesn't budge 1/10th of a degree.
Same happens for a neighboring moon, at a different temperature.
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I genned, deleted, and genned systems with SM as I set up one of my first custom games. (I plan on having a low-tech Trans Newtonian empire conquer several conventional start empires before heading out into the greater world.) One of these, a gas giant moon was listed as habitable in the System Info screen. I marked it as a SM colony and went to the environment screen to customize it, as I have others in the system. The other worlds could be adjusted, this one resists.
I do not have the worlds initial atmosphere. -- Grav 0.33, Orbital Distance 649k, diameter 6200. Its primary is a Super Jovian orbiting at 25.8b.
I just tested for a neighboring moon which stops improving at a different temperature. So, maybe, this is working as intended.
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I will hold onto the save for a week or two, so I can provide it and further details.