OK so the issue that is happening is that I have a standard boring gas vent which has gotten a room to 145kg of Hydrogen per square now (which seems to be the limit). It appears to be when water is occupying the exit square and it doesn't cause any side effects, the room doesn't explode with the insane levels of pressure, etc.
The screen show attached shows the square directly above the vent but the pressure is much higher elsewhere in the room.
Steps to Reproduce
Let a small amount of water into a room, have a standard vent on the floor, close the room up and let gas fill the room.
This issue has not been confirmed by a developer yet.