Raindrops seem to render in places regardless of if the ground is underneath blocks. This does not occur for every block, but is seemingly distributed through some sort of algorithm. This rain is not real, and cannot be used for conduit power or the Riptide enchantment,
However, for rain on the surface, it no longer seems to fall on every block with open sky above it, but only certain ones. This part is not just a rendering issue either: the rain legitimately does not exist on some blocks, even though there are no ceiling blocks to prevent it from hitting the ground. This prevents full usage of conduit power above water, and makes the Riptide enchantment more dangerous to use, never knowing when you'll end up not in a rain tile. The raindrops that exist on the surface do not always render beneath other blocks.

This issue has been somewhat detailed already in the following reports:

1. MC-180542
2. MC-180549