@norctune The city's water was supplied by the Aqua Augusta (AKA the Serino Aqueduct) which was the longest & most complex aqueduct in the Roman world until the 4th century. It was a branch network system rather than a single line running to a major city like almost all other Roman aqueducts. Another unusual feature of the Aqua Augusta was that most of it ran underground, making it difficult to maintain. Here's the link to the article I checked -
https://en.wikipedia.org/wiki/Aqua_Augusta_(Naples)
Here I'm just making some guesses because after the eruption, the branches that fed Pompeii, Herculaneum, & Stabiae weren't repaired because there were no cities left to supply. Any investigations into the disruption of the water supply would have been moot since no rebuilding was going to take place.
The aqueduct branches leading to cities near the coast ran through volcanic regions where ground movement was a known factor to the Romans (which is why this system was so costly & difficult to construct). We witness an earthquake in this chapter, & most volcanic eruptions are proceeded by earthquake swarms, most of them low intensity. Still, it's not difficult to envision a minor earthquake fracturing a section of the aqueduct, allowing the water to be diverted. Such ground fracturing not only allowed volcanic gasses to exit the earth, but could easily drain water & feed it into lower strata. Much of the steam vented during eruptions comes from groundwater getting drawn into & vaporized inside the magma reservoirs beneath the volcano.
If you accept the caveat that I'm not a volcanologist, this seems to be a plausible explanation for the sudden interruption of Pompeii's water service.