Water doesn't fully freeze.

I mined out some ice in a frozen ocean. 
Filled the gaps with water source blocks.
Now the water only freezes in these strips.
Coincidentally the rain/snow also only shows on these "strips".

Multiplayer server.
World Type = Default.
Seed = -9176820221198559886
Biome = frozen_ocean.
Coordinates = -4400 / 65 / 750

[Edit]
Link to world download https://drive.google.com/open?id=1mFyM0Kn-n6C1dPVuuKweZiMH-fbdwkIX