Water sources not turning to ice at Y120+ in Taiga biome

I built an ice farm with water sources at Y=123 in a Taiga biome. Some water sources will eventually turn into ice, others do not. The distribution seems initially random and all biome features indicate the entire farm is in the Taiga biome. There is no added light to the farm, so light cannot be preventing ice formation. The water sources that do turn into ice will reliably do so, the sources that won't reliably do not.