DuckDasterd!y

This is a good question. Sasuke didn’t break the rules…he just became so good that he needs almost nothing to cast jutsu. In the beginning, most jutsu need many hand seals because you have to carefully shape your chakra. The more you master a technique, the fewer seals you need. By the end of Shippuden, Sasuke could already use Chidori with zero seals. He had practiced it so much that his body just knew what to do. As an adult in Boruto, his control is on another level. He has the Rinnegan, years of training, and perfect chakra control. At that point, one hand sign (or sometimes none) is enough for him to mould the chakra correctly for different techniques. The seal becomes more of a small trigger than a full set of instructions. Other strong shinobi can also reduce seals a lot. Tobirama only needed one seal for Water Dragon, and many high-level ninjas stop using full sequences once they master a jutsu. Sasuke just took it further than most people because of his talent, Sharingan, and the amount of time he spent training. So it’s not really a plot hole. It’s the natural result of extreme mastery. The series just shows him at a level where the old rules barely apply anymore.