I was playing my long term survival world and had just loaded into the world after leaving it to use the bathroom. I load in and all entities are gone. My villagers who I had worked hours on, My horses that I had tamed, a pink sheep I had named Fluffy using a name tag. It even affected chunks that where not loaded. I think the cause could be something to do with memory error. In the photo I have provided it shows where some colleges where supposed to be. Also I have no experimental toggles on if that helps