Twisting vines not generated

Twisting vines appear to be completely absent from my world.

I've been searching for 20+ minutes. This is a pretty extensive warped forest biome and it looks nothing like what you see in this YouTube video for example. (two-and-a-half minute mark)

Here's what I see:

I've got no mods or any weird settings - just AMPLIFIED biomes enabled. This world was generated entirely yesterday and today (in snapshot version 20w13a/b).

Seed: 4500432789051034519

(Just enter the nether where you spawn in the overworld.)