Whenever it snows in Minecraft, trees turn white. But if you enter a different dimension and return, this is disabled and all trees will remain green no matter what.

Work-around fix is to leave and rejoin the world.

Steps to reproduce:

1. Go into nether or end
2. Return to the overworld and enter a cold biome, like a cold taiga.
3. Make sure it is snowing
4. See how leaves begin to defrost even if it is still snowing.