My mobs are disappearing in my Hardcore world for some reason I dont know why and how

I was making an iron farm and I named a zombie to keep it from despawning but it keeps getting invisible to me for some reason and some of my villagers do that as well, I still hear the mobs that are invisible and the zombie deals damage to me when I’m close but I can’t see him nor my other villagers. When I log out and re-login I see them until I move away from the chunks then they are invisible but I still hear them and stuff, can someone help?

Is it because I started the world on 1.16.1 and then changed to 1.16.4?