This happens most of the time if you go above the nether bedrock however sometimes it doesn't appear automatically if the chunk is not loaded.

Minecraft version?
Forge or Fabric?
Mod version?

Minecraft Version: 1.16.1
Fabric Version: 0.8.9
Mod Version primalwinter-fabric-1.16.1-1.0.0.jar

I cannot reproduce. Have you made sure that the nether biomes are blacklisted from winter transformations in the config?

nonWinterBiomes = "minecraft:the_end,minecraft:small_end_islands,minecraft:end_barrens,minecraft:end_highlands,minecraft:end_midlands,minecraft:nether_wastes,minecraft:soul_sand_valley,minecraft:crimson_forest,minecraft:warped_forest,minecraft:basalt_deltas"

In any case, should be resolved by https://github.com/alcatrazEscapee/primal-winter/releases/tag/v1.1.0-fabric