The Bug

Cave vines generated from the placed feature cave_vines replace the ceiling block they find entirely rather than placing right underneath it. While in my testing this only created unnatural generation with leaves (I reference the vanilla cave_vines feature in a custom biome definition), I can see how this could have undesirable effects in vanilla (e.g. if a cave vine finds its ceiling on a one block thick stone ledge, it would be floating in air...)

This issue does not affect lush_caves_ceiling_vegetation, because in that instance it's the moss replacing the ceiling (as desired) with the cave vines then placing underneath. This issue only affects the solitary cave vines coming from the cave_vines placed feature.

How to Reproduce

Create a custom biome definition (or change an existing one) to have cave_vines place in a surface biome with trees. They do also exhibit this behavior underground (in lush caves biomes), but it's often not as problematic there because stone is thick.

Fixing

I was able to fix it by adding this placement modifier before the biome check:

    {
      "type": "minecraft:random_offset",
      "xz_spread": 0,
      "y_spread": -1
    },