We are using Zenith 1.2.2-1.20.1 in a modpack.

Shortly after spawning, if the boss is not aggroed onto a player, it "disappears". We thought this was due to despawning, but I stumbled on a pile (literally, they were all stacked on top of each other) of bosses in a spot in the ocean. This seems to have been every boss that we did not fight.

So far I have only confirmed this in the Overworld.

ive seen this a few times with the skeleton bosses on top of spiders, they go to spawn chunks for us