Your question is Seasonality vs Product Issue Detection. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You are tracking daily completed rides on the Didi Chuxing rider app and see a decline right after a dispatch flow update. The series has strong weekday and holiday effects, so raw trend comparison is misleading.
How would you use time-series trends to separate seasonality from a real product issue?