Snow layers are unable to form or melt at light level 11, this seems to be an off by 1.

Simple tests:

1. /fillbiome ~-20 ~-5 ~-20 ~20 ~10 ~20 snowy_plains
   /fill ~-16 ~-1 ~-16 ~16 ~-1 ~16 glass
   /fill ~-16 ~ ~-16 ~16 ~ ~16 glowstone
   /fill ~-15 ~ ~-15 ~15 ~ ~15 air
   /fill ~-11 ~ ~-11 ~11 ~ ~11 snow
   /weather rain

   → The snow does not form 1 block further than the fill command, no matter how long you wait

2. (from same position as the other commands) /fill ~-12 ~ ~-12 ~12 ~ ~12 snow
   → The snow spawned in by the command does not melt, no matter how long you wait