There is a really fascinating phenomenon with the brain where once you understand how to solve a problem, the solution is inherently visible to you when faced with the same problem again.
This is applicable in a myriad of things. This is how we go from seeing the silhouetted faces to the wine glass or the bird and the old lady back and forth once we are aware of the optical illusion, despite only being able to see one or the other upon the first viewing. This is how we can “learn” to increase our IQ or our SAT scores by repeated test taking. This is how we become more adept at puzzles and games, and why you’re no longer play Sudoku.
There is an evil twin sibling of this phenomenon called confirmation bias. And this means that when we believe we are solving a problem we have already learned to solve, we will find clues that helped us solve previous puzzles that might no longer be applicable. This is why you never finished the “expert” level Sudoku book.