On my SMP server running 1.8.8 vanilla, 7 of every 9 (so far) named slimes have despawned.

Ok thank ya! So there is an issue with the nametags ok. I am using 1.8.2pre4 and it was a simple spelling error on my part. I spelled persistance <--- with an "A" instead of an "E". it is supposed to be PersistenceRequired:1 not PersistanceRequired:1. I changed it and it seems to be working now.

@TheTamedWolf: That's MC-27535

Oops I found an open ticket with this issue that you addressed, sorry about that lol.

I am in creative mode when it happens and the other person was in survival so that isn't the issue I guess must be something else huh.

EDIT: I never tried nametaging but for my map that would be ridiculous lol so it never really occurred to me. The persistance tag doesn't seem to work for me though. BUT I have noticed that if you spawn in a mob and it is successful, it will eventually DESPAWN because of poor coding. Like if you type an error, like if you forget the s after Damage:3[s]. The mob will SPAWN successfully and the command block agrees that it is so. However the mob has a genetic defect of sorts and dies soon after at an unpredictable time.

@[Mod] Kumasasa

Hi, I know that this was for a much earlier version but I seem to be having the same problem and I am in the recent snapshot 1.8.2 pre4. I am having this same problem except I am on Normal Difficulty. I thought PersistanceRequired:1 means that they will never despawn... ever?

If it is working as intended, how do I make my custom mobs not despawn I need them to stay for an experiment? They all are given names when summoned. So I originally thought that that made them permanent but they still despawn. If you could help me out I'd be most grateful.

There is no bug. Everything is working as intended.
Since you are cheating anyway, why don't you switch to creative mode instead to peaceful, that will preserve the loot.
If you want to have the game mechanics changed, please use the MineCraft Forums: Suggestions.

I also think this is intended. Peaceful just gets rid of all hostile mobs.

The problem isn't necessarily that they de-spawn, but that they take their pilfered loot with them. It's not unlikely for a person to die and then swap to peaceful to make getting their stuff back easier. I don't personally, but it's not something that never happens.

I have amended the report to reflect the issue more accurately.

Because they're supposed to!