Having the same issues. I started a career mode on World Class difficulty and after playing a handful games and losing or tying them all (Playing as Everton and most of the games being against middle or bottom of table teams) I decided to lower the difficulty to Professional. Suddenly all my opponents stopped playing defense and I was scoring 3+ goals in the first half every game (I thought it was maybe just a weird glitch the first game or the team I played just wasn't rested or soemthing, so I played a few). That wasn't fun at all. I want a challenge, but I still want to win games.
I don't know what's going on and it looks like it's not just me that's having the same issue. I'm hoping that they can fix this issue. It's been a few years since I've even thought about buying EA FC. I bought FC23 and it was straight trash. Tried a handful of times to play it and it was straight trash every time. I played FIFA 14,15,16,18 for more hours than I'd like to admit and even though they had their quirks, I never felt like they were terrible, or had game breaking bugs that never got fixed. They for sure never had issues with the difficulty levels being jacked up.
FC26 gave me hope that this game might actually be good after playing the trial. The gameplay feels great, and I enjoy the Realistic gameplay setting. It's just too bad I can't win on World Class and can't lose if I tried on Professional. Developers, I know career mode and other offline gameplay isn't your money maker, but please figure out this difficulty level issue.