Forge version: 36.2.2
Twilight Forest version: 1.16.5-4.0.515-universal
Link to crash log: (please use a paste site such as gist, do not attach the .txt or paste the log inline)

Steps to reproduce:
1.

What I expected to happen:
Lich not to float through the floating gravel
What happened instead:
Lich floated through floating gravel

This looks normal to me, the lich hitbox extends above 2 blocks so it's technically inside the third block, vanilla mechanics state you can travel upward through blocks you're already inside.

This is all normal vanilla behavior.

Try this with a wither skeleton.