Mobs spawned with spawn eggs disappear after some time. I'm not sure if it happens only when gamerule mobSpawning is set to false or if the issue is caused by some other game rules. My difficulty was definitely not peaceful - i successfully spawned the mobs, but then after some time they disappeared. And there is no way they died - they were held in closed rooms(some were blocked on 1 cube), there was nothing to kill them. Maybe the problem applies only to specific mobs, like skeletons and zombies, which i spawned(and they disappeared). Also, it all happened on a super flat world type. What's important is they disappear only when i don't see them on the screen or i am far away from them. Maybe it happened when the mobs were out of my render distance, but I don't think they ever were. I spawned them already a few times, I reopened Minecraft, restarted my computer and the issue keeps happening. I can't search for any setting in minecraft that might have something to do with this issue. Do they actually disappear on purpose or is that a bug? I'm trying to make a dungeon map and it would be easier if the mobs wouldn't disappear. I spawned the skeletons like in the image in the attachments, they just disappeared after some time.