What I expected to happen was...:
I expected Endermen to spawn on a platform over 128 blocks away from the island in The End.

What actually happened was...:
They stop spawning

Steps to Reproduce:
1. Go to the platform in The End
2. They spawn regularly
3. Make someone else enter or exit to or from The End while another person is still on the platform
4. Endermen stop spawning
5. Get everyone in The End to disconnect and reconnect to server
6. Endermen spawn regularly

Additional
This seemed to begin after additional layers were added to the platform. The new layers are using the trip wire style of enderman killing, while the old were using the pressure plate style. The platform starts at y 50, the old style goes to y 119, and the new goes from 119 to 148.

This is on a server on 1.4.4, I do not know what the Java version is.

Edit:
This occurs even with only 1 person in the end, only at the platform.

Edit 2:
Sorry for wasting your time, folks; there was some miscommunication on the server, and at the time of posting there was actually someone on the island. All is well and thank you for your help.