A bulk gas deletion system can be demonstrated simply by forcing upward mobile gases into this cap such that they are confined from moving up or side to side and confined from combining with gas packets below them; an alternative mirrored system with a vent on the ground to delete heavy gases is expected though I haven't tested it yet. The system is unable to displace the gas covering the vent and just deletes it regardless of mass or temperature. I do not believe this only occurs with high pressure vents, but I think it is easy to demonstrate with them.
Steps to Reproduce
I have three gases in a tall chamber that is 1 tile wide. At the top of the chamber is a three tile high cap with a high pressure vent. The snap shots show a packet of hydrogen gas bubbling up into this cap and being confined to the tile covering the high pressure vent. The vent then emits steam which deletes the hydrogen gas; I checked and tallied up the mass in the hydrogen mass to the left side, and the mass was not add to it; it is just deleted. Running the machine under various conditions of lower or higher gas pressure shows that the mass of the packet doesn't matter; it will as readily delete 100Kg of hydrogen gas as it will delete 1 mcg. The specific choices of gases is not really relevant here; this isn't specific to just hydrogen, steam, and carbon dioxide. Provided that the combination of gas choices result in a gas that rises above the others and the vent expels a gas that is not the gas or gases that rise to cover the vent. While these screenshots were taken when I had a few mods active, this behavior can be demonstrated in unmodded sandbox and presumably the unmodded base game as well.
This issue has not been confirmed by a developer yet.