Mob can't properly or difficult walk on big dripleaf, when the big dripleaf is 4 blocks or more from the ground

Can confirm

Pigs panic run, follow carrot, and random pathfind over Big Dripleaf blocks as expected (normally) for me. Unless you can show a video of what you mean that goes contrary to what I am seeing, I'm marking this as CR.

Yep, I'm also unable to reproduce this in 21w18a. Pigs can indeed walk on the big dripleaf without any problems.

Edit: This only occurs when the big dripleaf is 4 blocks or more above the ground.