Do you think you should have them roll a dexterity check vs the monsters Wisdom each time they go for that sneak attack or should they/the dungeon master just describe how the chaos gives them the advantage to keep sneak attacking?
Personally, I look at it that simply.
Player: "Who is the Monster focused on?"
Me: "The last who attacked it, Fighter."
Player: "I attack the monster as its distracted."
OK, thats a Sneak Attack.
Its just my view on it, but I've trained, sparred, and watched a silly amount of fighting in my life. It would not be difficult to land sneak attacks.
The other problem is also my players are also sort of expecting that mechanical hoop as well, and it seems like some of them just can't imagine a world where can just sneak attack in the middle of a chaotic battlefield or a character without dark vision can scout a head (to use another example).
Thats a perception question then. What they percieve a sneak attack to be.
To me? Its an attack that was not seen coming in. Boxers always say, 'its the one you didnt see coming that hurts you' well think about that. Trained fighters, in a one on one, 'miss' or 'do not see' an attack coming! If thats the case, how can we deny the thief, not even the focus of the monster, a sneak attack?
To me its not even a debate, its just 'yeah, most likely you get that damage'.