Steam temperature is negative 25 Celsius when geyser is idle

Yes, this can happen when gas concentration is very low. I don't know the cause but this seems rather intentional.

EDIT: However I do think it seems rather strange the steam has that temperature in an enclosed box. You should check the temperature of those neutronium tiles. If it seems unusual, the bug is more about neutronium exchanging heat while it shouldn't. That or the ice was there prior, idk if you put it there.

Edited October 13, 2024 by SpomJ

Elements require a thermal update from other elements to phase change. Elements <1g will not exchange heat with other elements. However, they do exchange heat with entities. Hence, exchanging heat with just entities will not cause them to phase change even if the temperatures indicate they should (This is also how you get food to be deep frozen in rock gas/gold gas)