The fact that it doesn't is largely unrealistic
pfft. Realism again.
The really unrealistic thing about it is not how it does or does not catch whatever type of water,
but that it automatically materializes a jar/bottle around the collected fluid.
Well at least that explains that the filter cannot be just crafted from plant fiber, sand and coal, but is a "find only" thing.
The filter must include some 9th-dimensional molecule converter, that sucks the jar over from a parallel universe, just as long as there's some fluid left in it, and then dematerializes it back to where it came.
Maybe that's the yet-to-be-revealed background story to where the zombies originated?
Some scientist tried to invent an infinite source of glass jars by messing around with opening gates to different realms of space and time?
...yeah well, I'd buy it. Sounds realistic enough for me, playing a game set in a zombie apocalypse.