I'm going to hijack this thread with my own information.
I ran into this problem recently:
I was terraforming Mars.
I had 10 terraforming installations and ~5 10 module terraforming stations in orbit.
I was adding oxygen with a max value specified at 0.1 atm.
At around 0.08 atm of oxygen the terraforming just 'stopped'. No messages, not notifications. I only discovered this because I manually checked the environment tab of Mars and saw that the gas dropdown was set to 'Terraforming Inactive' (or whatever the wording is; don't have game in front of me now).
I had to re-enable the terraforming for it to continue.
Note, I did NOT check the population status at the time, meaning, it is entirely possible that I was running my installations understaffed. Is it possible that the terraforming just shut down due to lack of workers? Again, I got not messages or notifications; and, I had ships in orbit that required no on-planet workers.
Also note, I am fairly certain the population was well above that necessary to run the ten terraforming installations. THey would have required... 2.5 million workers; even accounting for the service sector, agriculture, and the limited number of construction/mine installations I am fairly certain I had plenty of workers at the time.
Given all that, I would like to be confident that it was not understaffing that caused the issue, and as such I cannot figure out what else may have caused the issue...
I had ~200,000 wealth, so even if I was running at a day to day loss I would have been fine, I had plenty of infrastructure on the colony and it was not over-populated. I am NOT sure what the protection rating was, but am I correct in thinking that colonies under 25m don't have a protection requirement? (I may have made that up right now)...
Does terraforming require minerals? Mars only produces extremely limited numbers of like two minerals; that said I never got any insufficient mineral events regarding terraforming that I can think of, but, it is possible I missed one since I've been fighting Corundium shortages so those types of messages are pretty common and easy to ignore.
Maybe with that info we can figure something out.