Kosma Moczek (Kosmolot)

Does this bug affect new world generation, or only upgrading existing chunks?

This is a piece of nether brick.

Still present in 1.15-pre4.

Techman88 suggests a temporary fix for 1.14.4 with a command block:

https://www.youtube.com/watch?v=BJ0jzlzEhgo

/execute as @e[type=#minecraft:raiders] run data merge entity @s {Patrolling:0b}

Note that this will change the patrol behavior a bit, but is still better than extreme lag.