mobs randomly disappearing after logging out of the world

it started a few days ago after logging out of my world. when I logged back on the next day, my panda that I kept with me at my base had gone missing. there was one way in or out and it was sealed. it had a nametag and everything. the next time I logged back on, all of my villagers had gone as well. they were all there and kept safe. there was no way they could've despawned. this made me extremely mad as I had sat there for 3 hours getting villager trades and now they are all gone. the next time, my friend's parrot had gone too. It was attached to a fence post, and the lead wasn't even there either. this has to be fixed because now I have lost all of my progress.