Legendaries in Pixelmon 8.1.2 will instantly despawn after 5 minutes no matter what. I've tested this on a multiplayer Sponge server and singleplayer. They will despawn even if you're standing right next to them. If they are in a battle at the 5 minute mark, they despawn instantly when the battle ends.
In 8.0.2, there is no such feature and legendaries stay around until a player gets far enough from them that they despawn naturally.
I'm not sure if this was a bug or intended, but if it's intended, could I get some answers on how to disable this feature? Or maybe add a config setting to toggle it for the next version?