Rain seems to be missing in several random spots, and looking straight up seems to display a much larger "rain gap" than there should be, by several blocks. I discovered this because while it was raining, an Enderman was standing just fine and taking no damage while standing in a rainy biome. If he wandered off of the block he was on, he'd wander into where rain was and teleport away, taking damage as he should, until he randomly teleported onto another block where rain was missing. When daytime came (and it was still raining), zombies and skeletons would start burning when wandering onto certain blocks, making evident that rain is missing in many places. I made sure to check that all the mobs were still in a biome that could have rain, and they were. 

I've also noticed random splash particles high up in the air during rain, as if the rain was hitting blocks that aren't there. I'm not sure but they may coincide with where rain is missing on the ground. It's almost like some air blocks are randomly being treated as solid by the game, except for the fact that undead mobs can still burn in the sunlight when in those areas.

I've attached screenshots of the bug, though it's a little hard to see. One shows the splash particles in midair, one shows some of the missing spots where rain should be, and one also shows the large, randomly-shaped rain gap above the player.