Liquids are not pregenerating

The bug

In previous versions, liquids would pregenerate rather than flowing down in front of the player's eyes.
This can be best observed in the nether since lava is going to start flowing down from the ceiling. In the nether, that also makes the game lag a lot after visiting for the first time because of all the light-updates being caused by updating lava.

This bug is probably the reason MC-123150 is no longer happening.