All mobs, banners, and paintings disappear every time I leave the world. I hit save and quit every time so it's not that I didn't save it

I have 2 worlds I play on most of the time, each time I join one world (a larger mansion) all paintings, animals, and banners are gone. The other world (a small house) all animals and sometimes blocks will disappear. This has been going on for 3 months now, this all started when I went into the world with the mansion and everything had been destroyed (all dirt, grass, and bedrock included)