I don’t have a video, and my upload is bad enough that i wouldn’t want to kill my internet for a day to do it. I’ll try my best to give example scenarios. If you have a pyro its pretty easy to test this stuff.
Basically Fireballs don’t follow the rules of normal aoes. Normally aoes would prioritize targets based on the highest aoe defense ratio of enemies. Which means the fireballs exploding endlessly on themselves would only occur when fighting against aoe defense ratio 1 mobs. Since the fireballs themselves only have an aoe defense ratio of 1. However it becomes immediately clear when stacking a ton of fireballs against a boss this isn’t the case.
An example of how it really works would be if you have 2 medium melee mobs and 1 large ranged mob. Placing one fireball on the ranged mob, and then moving a bit away so the two melees are you closest enemies causes only the two melee mobs to take damage(Fireballs have a locked aoe attack ratio of 4). Even though the ranged mob is literally standing on a fireball and has higher aoe defense ratio it will still prioritize the melee mobs next to the caster.
Another example is a boss fight. If a pyro stacks 10 fireballs then pushs them all into the boss. Its going to do almost no damage. Since in that scenario only if the boss moves through the fireballs to be closer to you is it going to start doing damage. Alternatively if you have someone else hit the fireballs in while you stand on the opposite side of the boss. All of the fireballs will deal maximum damage.
3rd example is when i play my pyro3 thaum3, I already have to deal with this positioning all the time. Since i shrink mobs down to aoe defense ratio 1. The difference between me hitting 3 or 4 mobs depends if i can get a mob to trigger my double fireballs while im closer to them while the other ones surround me. If i can’t get it perfectly I need to distribute the damage to those with the highest hp. So i don’t end up with 3 dead mobs and one mob at 3/4 hp. So i move closest to the current highest hp enemy to make sure it gets hit.