The end keeps reseting

Hi, me and my friends have been playing on an aternos server and have recently killed the ender dragon. However, when we returned to the end, it had completely reset and the ender dragon had returned. After killing the ender dragon again, the end reset itself for a second time. Does anyone know how to fix this problem and stop it from happening?

We're playing 1.16.3 on vanilla. Most of us are using optifine but apart from that there is no mods or plugins.

The server address is interrailinggang.aternos.me and the last time we saw the original end was around a month ago but it's reset like 5 times since.