Biome color change is abrupt

Biomes change color too brusquely: they change color within 4 or 5 blocks and tall grass/fern/vines and sky color don't fade at all. This issue is present since Anvil file format was added and it's very noticeable when there is a river in the jungle biome. I downloaded 12w03a, when jungles where added, but world used the old format, so i have a bunch of screenshots for comparison (the first and the last screenshots are from 1.4.3pre, the others are from 12w03a).