There is [still a] hidden bias in the passive mob AI causes them them to wander from their supposedly preferred terrain (grass) and then get 'locked in place'.
To reproduce, pick an island in an ocean biome, or some grass on the edge of a desert. (Or a grass island in the middle of the desert for clearer results.) Populate it generously with passive mobs. The more you use the more the bias is apparent. A few stacks of mob eggs should do.
Start mining, or just moving about in the area. As you move about, the mobs aboveground that are still in range will wander. Some will wander into the sea (or desert). Once out of range of grass, they will get 'lost' and wander randomly. As they're surrounded by water (or sand), with grass in only one direction, the probability is that wandering further at random will tend to make matters worse.
Eventually you can find them so far out at sea they're out of sight of land! The screengrab is a max-zoomed map, marking my original island, and how far out at sea I was when I found some chickens swimming about. Not just one, there were several just in that one area. Around the whole island perimeter I estimate 40-60 chickens had made it out to that kind of distance. Splashing about in the shallows is one thing, but that's 200-300 blocks, which is ridiculous! What's more, once they're stuck, as soon as the player moves out of range again they freeze in place, which means they can't get back to shore again.
The result is a very considerable built-in bias that causes passive mobs to venture further and further and further into unsuitable terrain, most noticeably desert and sea - and also down caves. Even with the northwest bug fixed this still happens, it's just much slower.
Note also that the bias is [still] cumulative. It's not particularly notiecable after a few minutes, but the longer you operate in an area the further the mobs will wander. And once they do blunder somewhere inappropriate, either they wander raondomly, which means the odds against them finding their way home are low, or else the game freezes them there so they can never get back to suitable terrain again!