It's always been this way. It also makes that sense the bars fall - a bit of rustiness is to be expected.
I do think it would make more sense if the change was dependent on how much your team has changed though (AFAICT this makes little/no difference), depending more on the number of players you sign etc. It seems a bit daft that the team completely "forgets" how to play the formation they've been using for three years.
Well, how does it make sense that a team that has played a 4-4-2 tactic for around 10 years, can suddenly forget how to play it every season to the point all of the bars are at "awkward" even after it being on very high match prep and after having played six friendlies? because it does happen sometimes. It's not a "bit" of rustiness, it's lots.
I also think this is really odd. I bring in only one player in my last offseason and suddenly my entire team forgets how to run my tactics? I don't think my tactic bars maxed out until I was into the second month of the season.
On my current save i signed tons of new players on the end of the first season so i assumed it was normal, then on the second season i kept most of my team and only signed a couple of players and they were still reset so i was wondering what was going on.
Just bumping this to say i started a new save in the meanwhile and now on my second season despite signing a lot of new players, my match preparation didn't reset at all while on my previous save it reseted every season.