I built a chamber around this steam vent. The chamber pressure was pretty much equalized with the rest of the base during construction.
It has been shut since (only a few cycles), and hydrogen pressure spiked.
Branch: Live Branch
So oxygen levels increase if surrounded by oxygen, hydrogen increase if hydrogen is present. Another observation, I created a vacuum around the cool steam vent and it is still surrounded by vacuum (ie, no hydrogen as last active period).
So, my guess is there is some bug with the gas compression algorithms causing gas to be reproduced rather than conserved.
I also have a feeling this is related to a bug I saw with excessive CO2 being generated near steam production of a liquid aquatune