https://github.com/Prospector/Traverse/blob/master/src/main/java/prospector/traverse/world/biomes/BiomeRockyPlains.java

Looks like the Traverse Rocky Plains biome is just outside the 0.8F cut-off for season change, its default temp is 0.9F. Not sure what Traverse's developer intended for snowfall for the biome, but I filed a suggestion on the Traverse issue tracker as well (Prospector/Traverse-Legacy-1-12-2#37).

There's really nothing we can do about it on our end. 0.8 is our baseline between snowy and non-snowy, and that's not gonna change. They register the Rocky Plains as a cool-type biome though, so the temperature probably should be changed for accuracy's sake.

Thanks for the info, @Forstride. Hopefully @Prospector agrees!