Oh, I'm not sure then, none of them have despawned on my server, even when no one is online
This has always been in Minecraft
If you name the horse with a name tag they they shouldn't despawn
It's spelt perfectly correct for me in 1.6.1.
It works completely fine for me, make sure you have a water source at least 4 blocks away from each seed, otherwise they pop out because they ground isn't hydrated.
The server is outdated that's why, just wait for the server to be updated. It should be updated within the next week 🙂
I have exactly the same problem! I was about to create a new issue for it, lucky I searched before hand, I tweeted my screenshot to Dinnerbone, hopefully he does something about it! xD