To put it simply, damage prevention for blocks (including: scaffolding, lava, vines, ladders, twisted vines, weeping vines and cobwebs) is unpredictable and rarely works. I believe this is something to do with the game only checking for these blocks every game tick, which causes problems when falling over at 20 blocks/second.

Steps to reproduce: Fall repeatedly from multiple different heights onto any of the aforementioned blocks

Observed results: Player fall damage is not correctly reduced/cancelled