After starting a new map in creative, I found myself next to a biome boundary between a desert and a savanna. After a little while, it began to rain, with some odd effects. The rain seems to only affect the desert, and only near the ocean or river areas. There is a river in the savanna that also has full rainfall, whereas the grassy area has none. The desert also has rainfall near the river. My guess is that the beach biomes are pretty much universal, with no regard as to whether it is in a desert or savanna, or other area. The river technical biome may be causing rainfall within the desert area as well, registering the nearby sand as if it were a beach.
After getting some initial supplies, I changed the mode to survival, so this probably affects both version. The world itself is in the latest snapshot as of 9/24/3013. The overall effect seems of minimal importance, even though it can be immersion breaking. Still, I didn't see this bug reported anywhere, and thought it might be useful to know about.