I don't think it's the tactic or updates mate. It happens to me as well with my Dortmund team unfortunately. Up until February / March, the team is performing brilliantly & then all of a sudden, they're playing like Amateurs! Some say because the opponents (i.e. the Match Engine) 'learn' how to deal with your tactic. But for me this doesn't make sense at all. I believe it's more something sort of built in FM to make you swear & bang your head to the wall!
Happened to me with different good tactics like this one, Twister & many others. Now, what I tend to do is keep another 2 'known good' tactics in the slots, like for example Conqueror & when February / March arrives, I usually switch to them & generally, I manage t'o keep some sort of consistency. Not always but anyway.
This thing has been driving me crazy for a while tbh & I'm glad to hear it's not happening to me only!