Huh? Your explanation is not complete for saves with no negative pressure. At start, that box with lava has no pressure. Then it suddenly starts generating loads of pressure. That part still remains unexplained.
Just to explain, some elements modify pressure in their own area by default.
For example, place a large block of Ice. It sucks the air in, right? That's because Ice element is literally coded to lower pressure slowly around it.
...Lava is exactly the opposite. The game's code tells it to increase air flow. It's not new, a glitch, or undiscovered. That's what it did in the original version before we made any changes to it. ^^;