Upon world loading in single player survival using the Mac Java Edition version 1.15 Pre-release 6 when my player is in the nether at the time of world loading, the overworld rain animation can be seen before the nether loads. I copied the world and loaded it in creative mode to go between the overworld and nether after setting weather to rain, and can confirm that this occurs when it is raining in the overworld, not when weather is set to clear.