Blocks/items/entities get deleted when downgrading a world from 20w51a to 1.16.5-rc1

Tried to take my old world into the new snapshot and it deleted some of my blocks, animals and new ingots. It completely deleted my new amethyst biome and all blocks in my chest of it. It also deleted all my cooper ingots that I had and then some how deleted all of my chickens in my farm.