The bug
Walls don't update / change their up: true tag on more than 512 wall height
To reproduce
Load the custom world in the attachments
Create a tall pilar of water with the command /fill ~ ~ ~ ~ ~514 ~2 stone_brick_wall
→ ❌ The lowest 3 walls in the middle row aren't updated, still have the up: true tagUpdate the lowest middle not updated wall
Go to the tallest wall /tp @p ~ ~514 ~
Place a wall next to, or on top of the top middle wall
Go down /tp @p ~ ~-514 ~
→ ❌ The lowest 3 walls in the middle row aren't updated, don't have the up: true tag
Can confirm in 21w06a.