The AI doesnt adapts to the formation and the mentality.
The AI doesn't "adapt". The game doesn't have learning AI. What happens is that at mid-season, you side is "re-assessed". And if you are doing well, opposition AI will view you as more of a threat. The odds change. Teams that were attacking you and leaving space are now sitting back more and that space is gone.
Think of it like Leicester. Their title season, they were relegation favorites. Everyone attacked them. They defended well and had endless space to counter attack into. Some teams adapted, and some did not. In FM, more teams would adapt and start to see them as more of a threat after mid-season, meaning that this style would be a bit less successful. It wouldn't necessarily fail entirely, because defending well is still defending well.
If your mids are just passing to each other and that's it, they lack options. This could be because you have very short passing set but players aren't close enough, it could because opposition are packing the midfield with bodies and taking away passing lanes. It could be that you don't have enough attacking options. It could be that those attacking options aren't moving off the ball enough so they are getting marked too easily. Watch the matches. Stop and analyze. When those passes aren't going anywhere, why not? What legit passing options did they have? Why did the player not take the "better" option? Could be a team instruction, role, duty, or even a change of shape that's needed. A common mistake is to use a really high line and then have problems breaking down the opposition. You've compressed them into their zone, which actually makes it easier for them to prevent good shots. Sometimes pulling back can be really effective.
leicester arent a good example, they cheated to win the title. arsenal figured them out and was the only team to beat them home and away that season