how big is the drop?
i usually experience a small drop after aquiring new talents too, and 2-3 weeks of rising merch income after it. Since i believe the fans are not common to the new stars,and need some time to be excited about them. Another theory might be, that they play lower division before(which is mostly the case for me), and the fan look at there effort of the last 2-3 weeks where they played less important games. Even when i would expect a little raise, when you just have an aqquisition, without selling dudes like you did.
The loss through a lost cup amtch is btw pretty small.