Ngl I stole this from MrLlamaSC and BTN.
The way the game works is it loads an area of 1.1 screens. If any person steps into an area, it loads that area. When you leave that area, there is a grace period of 8 seconds before the area "unloads". When an area unloads, all corpses and opened chests despawn and all modifiers the player applied on monsters like prevent monster heal stop working. So, if you kill some monsters, then go far away from them and wait for 8 seconds, if you come back you will find no corpses. This is done because of memory issues. If everything was loaded all the time, the game would be unplayable.
This is why when Baal summons a clone in vanilla D2, you go to town and remain there for 8 seconds and the clone despawns.
Your town portal counts as a person in terms of loading the game however. So, if you for example spawn a Diablo Clone and apply prevent monster heal on him, if you open a tp within a distance of 1.1 screens from him, when you go to town and come back, the prevent monster heal will still be on him.
For Baal if you open the tp near enough, the clone won't despawn.
This also allows you to drop "extra" items in chaos sanctuary. Normally, you pop the last seal and kill the last seal boss(if any). This kills everything in the chaos santuary but monsters you don't see don't drop items. But if you open a tp near a bunch of monsters you can't see, they will still drop items when you pop the seal.
One more application of this in the quest where you rescue the Barbarians in Act 5. Normally, you have to wait around for them to go to town and they are slow and it's annoying. But if you open a tp next to them, they will go to town without you having to be there. You can go as far away as you want and they still go to town.
I am not sure if there is a way to exploit this in MXL or if it is even a thing in this mod. In Rathma Square for example, the gore crawler ressurection does not require the corpse itself in order to work. That's why they still ressurect even if you kill them with cold attack and break their corpse. But I thought it was interesting.