Sometimes the new caves' tuff layer is visible at ground level.
I also noticed that when new caves spawn in rivers (it's kinda weird), part of them don't generate and they are flooded.
Seed: 8908892175142412717
X: -200 Y: 70 Z: 10
Environment
Java 1.8.0_201, Windows 10 Home
Hey there!
This looks to be a duplicate of MC-203636.