I've seen this bug across several bug reports. It seems that Cool Steam Vents produce amounts of gas surrounding them together with the steam they regularly produce.
In my colony the cool steam vent was surrounded by hydrogen and I found it a good way of cooling so I let it be. But every time it overpressured and the vent stopped producing steam. Several hundred cycles later, after a little bit of automation, I filled a fool 20kg room of hydrogen and I started filling a second one.
So I experimented with an untouched cool steam vent, cleared the room and filled it with almost 200g of hydrogen. This is what happened.
Here is the other same bug reports I've found
Steps to Reproduce
fill the area surrounding a cool steam vent with at least 200g of hydrogen (or probably any gas) , let the cool steam vent work and see the hydrogen multiply
This issue has not been confirmed by a developer yet.