Small slimes still sink?

Its quite simple really, make a large body of water and fill it with slimes.
The smallest ones sink as usual, but the medium and large ones will swim to the top. I assume this is part of the "AI Changes" added in this snapshot.

(Water is inside of a barrier "box" in posted images)

After reading the wiki i see they added swimming.
So is it a bug that the little ones still sink?