I started up a world, and noticed there was a partially submerged cave nearby. I noticed that the water was not flowing down into the cave. I took a look, and noticed that the water was floating over the edge of the cave and not falling.
MC-84152
Water does not flow properly upon world creation
Resolved
Duplicate
0
Calan
(Unassigned)
Unconfirmed
Minecraft 15w31b
Retrieved
This is because blocks need an update. When they are generated, they don't update on their own. Much like "primed falling sand" needs a block update before it falls.
This is not a bug. This is a way to prevent the client from freezing by scheduling block updates for every generated block in a chunk.