I know from testing this myself that on void affinity CB, there are several factors that go into determining the Stun Target. I was only able to successfully manipulate CB into stunning the correct champ about half of the time as I made changes, however...
He will attack the champion that the AI determines he can kill the quickest.. or off just the one stun depending on what round it is and how much damage he has built up.
Void CB seems to stun Spirit affinity more often than other affinities when youve got multiple champions of differing affinities.
Buffs like passive shield from Nekhret the Great, or increase defenses and Ally protect do not seem to change the AIs calculation and even when making the initial stun target much stronger via any of the aforementioned buffs, CB will still stun same target if the AI is going off champion that he can kill the quickest.
There are other factors the AI considers as well, and CB could change target if any champion Hap drops considerably, compared to the rest of the team's HP pool. Void affinity CB tends to target your reviver if youve got one in your comp, atleast this happened to me 90% of the time.
There was something floating around about a year ago among the content creators that the mastery in support, max health +810, would make the CB not target that champion anymore. I tested this aswell, and found that only once could I get the CB to not stun High Khatun after changing to this mastery, and this happened after also changing her turn order and a minor adjustment in her decense value.
The mastery in support, im sorry im not by my computer at the moment, the mastery that gives a 25% to remove one debuff if incoming damage exceeds 50% of champions max HP, does indeed remove the stun right away. Block debuffs also blocks the stun, atleast it does for me.