I found this bug where when there's something glitching on a spigot multiplayer server, for example spawning on the same block after sleeping on beds that are put on top of each other, or when duplicating a dragon egg through the end portal etc; after a couple of seconds all the entities spawn to us. I'm talking about over world, end, nether and everything. It happened to me multiple times already, making my weeks of work on villagers and all the mobs spawn in front of my eyes. After this happening we tried to do the glitch again, and it worked, which gave us the chance to record it, so if needed I have video proof of it happening. I first went to the Aternos team, telling them all of this, and they lead me here. I included the list of plugins we were using.
Here is the log link, 492nd line is when it happened.