After huawei mate 9 upgraded to Android oreo, the LED light no longer flash if missing a call.
Oreo seems quite different from Nougat, you need to turn on "pulse notification light" in app setting, but I didn't find it in zoiper setting.
FYI, GMAIL notification light setting as below
Go to Settings > Apps & notifications > App info > Gmail > Notifications > UNDER Miscellaneous tap on "Miscellaneous Make sound" and a new window will open and in here is where you make the individual app adjustments where you turn on "Always pulse notification light"