I don't know if the rules have changed (or if it was a state or federal rule), but when we lost water pressure due to a hurricane, the boil water advisory had to be maintained for 72 hours after pressure had been restored purely for regulatory reasons.

(This isn't a knock on the regulation--it actually makes a lot of sense. A loss of water pressure means that groundwater can seep into the water pipes where previously clean water was leaking into groundwater, and it will take a couple of days for any such intruded groundwater to be flushed out of the water pipes. This is merely me trying to point out that a boil water advisory being maintained after a pressure loss doesn't contradict the possibility that the problem has actually been fixed.)