Using vacuum as an insulator I've found that gas randomly gets into my vacuum. It's been happening in the save at random times - reloading the last cycle and replaying it would change the outcome and my vacuum would not be compromised.
However, I finally got a reproducible savegame - note the little bit of gas(CO2)/heat in the upper right corner, that's where it shows up every time, and in this save it triggers when my dupe opens the inner door to the power unit.
I reloaded the save 5 times getting the exact same result - probably because the dupe already had a target inside the power unit so he will enter at the exact same time each reload. If I manually move him somewhere else - and then inside the power unit no gas are created in the vacuum, so it's clearly related to his entering through the door. I hope this can help explain what is happening.
Steps to Reproduce
Load the save and let it run keeping an eye on "Harold" as he runs into the vacuum insulated room.
This issue has not been confirmed by a developer yet.