Spawners in dungeons stop working In realms when another player goes into the nether

When I was at the spawner in our world killing zombies and getting xp on my realm. One of my friends went through the nether portal and the spawner just stopped working. I had to use a backup to get the spawner working again. And we lost a bunch of progress. This has happened to us multiple times now. And is getting more and more annoying. I hope this can get fixed soon because it means we can't go to the nether.