Mobs Disappeared Not Despawned

It started when I joined my single-player world. Straight away I saw my llama "died" after seeing a carpet and a leash laying on the floor. Soon after building his grave, I went outside to feed the cows and the chickens but all vanished. Every animal that I owned disappeared. It is not despawning because it is impossible for everyone to despawn. I tried searching it up, but it came up with despawned animal videos.

Thank you.