All entities dissapear when loading a world in an older version if its saved in a newer version

I had an iron farm, a villager breeder and a villager trading hall. I had the world las loaded in 20w51a, and I have now installed optifine 1.16.4 but all the villagers are gone. When i loaded the world back in 20w51a, the villagers were back. But now I cant use my villager breeder, iron farm or my trading hall if I want to play with optifine. (Wich looks much better.)