So, I have seen people bring this up as an issue, but I play on a Realm with five other people (six total.) Almost every time that I have gotten a pet- it ends up randomly despawning. No death in the chat, no hostile mob possibilities, and no clear reason for the disappearance. None of us have ever crashed on the server and we've had it for months! I've used nametags, leads, and have had them in enclosed areas, but when I come back from... say mining- they're gone. Lead and all. Now that 1.17 is out I got a few Axolotls but I'm wondering if it's even worth having them since I'm worried they'll just despawn anyways. Since the beginning of the Realm I've had a donkey and he has stayed the entire time we've been playing, but I've lost- a cow and chicken (twice- both with nametags, enclosed area, and leads), a baby slime (nametaged), a polar bear (nametaged and enclosed area), a brown panda (nametaged, enclosed area), an iron golem (but I'm willing to chock that up to it dying since... well that's pretty much its purpose even if he was nametaged), and others. It has been a bit frustrating because... well... who doesn't love having pets? I've loved being able to have interesting and unique pets like pandas, polar bears, and foxes (slimes too), but it just hasn't been working out.

Now, I thought that maybe it's because I have a ravine base and the mobs possibly recognize they're not supposed to be that far below so they despawn, but I've had two mooshroom cows at the lowest point of my ravine and they're still there with no nametags- just in an enclosed area.

My next theory would be that they just despawn when they're not loaded in the chunk, but I don't think that makes sense since they would technically just be in limbo until I got back- or someone else would have them be loaded in since it's in the middle of our base. Some pets that actually have not despawned is a dog that I've had for a while, my donkey, a horse, and two mooshroom cows.

Now, we do occasionally use creative mode to do large projects, but that's about it. I thought maybe it's the switch between both creative and survival that screws them up, but like I said- some pets/animals stay.

I'm pretty confused by it all and I'm not sure if it's just a realm thing, but I was hoping someone could answer it or at least give me a theory so I could work around it. If not, it's really not a big deal since at least our farm animals have stayed- the pets are just a fun bonus. 

Thanks for reading my ridiculously long message! Have a great day and thank you!

Just a quick memorial for my "fallen" solider pets- Iron Golem (Irony), baby slime (Hutch), Polar Bear (Butters), Cow (Eleanor), Chicken (Alice), and brown panda (Brownie). May they rest in peace wherever they are at lmao