When cave vines generate too close to the ground the block below them gets replaced with stone/granite etc. while it should be clay or moss, this is likely unintended behaviour.

How to reproduce:
Create a large biome's world with seed: -240861352427657586
/execute in minecraft:overworld run tp @s 767.80 16.50 854.78 -487.20 47.25
/execute in minecraft:overworld run tp @s 724.50 25.00 772.50 -350.70 24.90
Might take a few attempts due to 219200

Note: To create a large biome world you need to create it in 1.17 first then upgrade it in 21w39a