Snow generates above Y=~570 (Shattered Savanna Plateau) or above Y=~630 (Shattered Savanna) as the temperature drops below 0.15 at those altitudes. However, both shattered savanna variants are dry/warm biomes so no snow actually falls even if the weather is set to rain or thunderstorm.

Seed: 5580744162200595349
Co-ordinates: 4500 800 300.
World Type: Custom (Size_vertical set to 8 to allow the shattered savannas to reach those altitudes)

The screenshot is taken in 21w05b, but should affect newer snapshots too. The screenshot only includes Shattered Savanna Plateau, but there is a Shattered Savanna next to the Shattered Savanna Plateau.