The bug

When rain or snow fall all the way to the bottom of the world and hits Y=-64, the particles disappear. This is inconsistent with water droplets, lava droplets, and all other particles, which do not disappear after Y=-64.

How to reproduce

Create a default superflat world.

1. Dig four blocks down in one spot (through the bedrock floor)
2. Set weather to rain or thunder
3. Notice how the rain stops at Y=-64

1. Dig three blocks down in a different spot (just above the bedrock floor)
2. Place a water bucket or lava bucket down on top of the bedrock here
3. Enter spectator mode by running /gamemode spectator
4. Go beneath the bedrock, and notice how drops do not stop at Y=-64