It appears cave vines can generate attached to any block as long as it generated before it in the generation stage. This can lead to cave vines growing from cobwebs and potentially other transparent blocks.

Example:
Seed: -8508883206669806165 (lush caves simgle biome world)
/execute in minecraft:overworld run tp @s -39.93 -43.00 252.42 -812.72 25.59

This might be hard to reproduce because of  MC-219200