I Was Recently In A World And Spawned Villagers, And Once I Spawned A Cartographer Villager, It Froze The Server. Mobs Didn't Move, The Inventory Didn't Pop Up, All I Could Do Is Move.

It Is Only Happening On Certain Worlds.

I Don't Know How To Produce This, But I Think You Spawn Around 30-50 Mobs And Spawn A Cartographer Villager.