Twisting vines don't prevent deadly fall damage when placed on top slabs

What I expected to happen was...:
For the twisting vines to prevent fall damage since they do when otherwise placed on normal, full blocks.

What actually happened was...:
When i fell from high altitudes and landed on directly on the twisting vines, i died from fall damage: "Fell from a twisting vine".

Steps to Reproduce:
1. place a top slab with nothing underneath
2. Go into survival mode
3. place a twisting vine on the slap

4. fall directly on the twisting vine from over ~30 blocks (enough to kill an unarmed player from fall damage)