Liquids don't spread out instantly anymore when a chunk is generated

In versions of the game before 1.13, water and lava used to spread out instantly when the chunk they were in was generated, this however doesn't happen anymore now. This issue is particularly noticeable in the Nether, where lavafalls will slowly flow down, as seen in the attachment below.

How to reproduce:

Go to the Nether, change your gamemode to spectator, then start to fly around to generate new chunks and keep an eye on any lavafall that might generate.