vacuum spot forming in sour gas boiler despite gas pressure

I built a sourgas boiler with no supercoolant in a sandbox save. now as its running, I am noticing a weird phenomenom.
In the last counterflow section there is a vacuum tile forming where the sour gas condenses (so far so good), but it persists. the gases from the tile above and below it do not move into the vacuum tile, which leads to the curious situation of:
40 kg gas pressure (sour gas)
vacuum
1 mcg gas pressure (sour gas)

The situation resolves itself after reloading, but only for a moment before it again, with the vacuum tile forming on the lower end of the last shaft and moving upwards tile by tile.

The miniscule amounts of gas below the vacuum tile also dont exchange heat with the -230°C hydrogen thats running in radiant pipes.