Poison Storm & Infectios Cloud (Shaman)
 

Poison Storm hits a single target with 3 waves of damage, but if there are more targets added, it will go down to 2 waves cast, and if even more targets are added, it will go down to 1 wave cast, and will not hit them all.

Infectious Cloud also hits a very limited number of mobs in an area, such as 3 out of 5 that are all clumped on eachother.

Is this intended? It seems that ALL mobs should be hit. And it seems that ALL mobs should be hit for ALL 3 times with Poison Storm (but it doesn't).

Rain spells (Poison Storm) have a max of 4 hits (No idea why its like this)
-- 1 Mob will get hit four times
-- 2 Mobs will each be hit twice
-- 3 Mobs and one gets hit twice and the other two just once
-- 4 Mobs and each just gets hit once

Some classes have rain spells that will hurt the caster if they are too close to the target. Rain spells are just not that great in this era

Not sure about Infectious Cloud, Im not too familiar with that one

This is how AOE Rain spells (Orange Spell Gem) have always worked.

It hits for 3 waves, but it has a maximum number of targets hit = 4 (you can, however, hit yourself all 3 waves even if the NPC limit of 4 hits is met)

This means; as stated by Kavious above:

1 Mob = 3 hits in a row
2 Mobs = 2 hits each (2x2=4Max)
3 Mobs = 1 hit on all 3, then 1 additional hit on the first mob in the stack (first mob is defined as the first mob hit on the first wave if they haven't moved and "rearranged" their order) (1x3+1=4Max)
4 mobs = 1 hit on all 4 (1x4=4Max)

Note: before Velious launch, pets used to eat these charges and report the spell immune text, so at least the patch were Verant fixed pets to not each charges from rain spells is fixed, before that these spells were almost entirely useless on P99.