Locking a cool steam geyser in a room inexplicably causes more of the other gasses to spawn. As an example: in this screenshot you can see this sealed chamber has managed to build up a pressure of 5kg/s of oxygen making the geyser block up. The pressure only seems to go up when the geyser is errupting.
I have encountered this phenomenon in multiple bases, it seems to not be limited to oxygen either, within the same base I have another geyser that spawns hydrogen for no reason and in another save I had one that flooded with Polluted oxygen over time.
Enclose a cool steam geyser in a room, condense the steam and watch pressure rise.