if you have stabilized spawners on an island after doing /is restart it will keep the spawners there but the spawners there aren't really there, they just continually spawn mobs without the spawners Screenshot by Lightshot
So since Slind created a bug report and the issue sorted itself there is nothing more we can do Just make sure to restart your island some time before a restart. Gonna set it on Done since whe know a way now to get rid of them. If not feel free to mark this thread as Open