I would guess that's one of those "pick any two" things, given how many construction projects, repair, "life happens" stuff goes on in a modern city
That said, unless I'm totally missing something they have a self-solving problem with that since the Waymo's all carry around cameras, lidar, and presumably radar so I would expect that they update the maps as they go. Come to think of it, that's very likely why I originally saw them roaming around the city with drivers in them: testing the pathfinding and mapping at the same time
I would believe at some point this is possible though, since at least if they send the data home about things that "don't match" with their precise maps, they can easily use as much compute as they want on the servers to get the maps updated.