I refuse to believe that the AI 'learns' your tactic because that would be stupid. IRL, top managers have used the same tactics over and over again and they don't just suddenly start failing because people know about them. I had a PSG save that I played up to 2020 before I got bored and I used the exact same tactic the entire time and dominated every single season, not once did I have a massive drop in form.
A better theory that I've read about is that the teams are reacting to your increase in reputation. In the past, the top teams didn't see you as a major threat so maybe they would have used a very attacking style against you and you were able to punish them for that, maybe by hitting them on the counter. Now that you've finished near the top of the table for several years in a row, these teams are starting to respect you more and so will play more cautiously and rather than being on the receiving end of a counter attack, they are the ones doing it to you.
My guess would be that there's a flaw in your tactic. It works great against weaker teams or ones that play very offensively but maybe it has a hard time dealing with top teams who aim to hit you on the counter. To fix this, you would need to look into what kind of play is causing you trouble. How are you conceding goals? Long balls over your defenders heads? Crosses into the box?