I had it two times now, that a fire occured. A well was nearby (about 10 tiles away) and a lake as well (about 20-30 tiles away). Half of my men got water from the well (4 water each), half from the lake (10 water each). So far so fair, maybe the well does not have enough capacity.
But half of my well-men go to the lake directly after they got the well water and get some more water. (up to ten). The other half of my well-men put he fire out as i expected.
Seems only to work, if a lake is in reasonable range to the fire. If only wells are nearby, this does not occure.