In MIUI update to 9.6.1.0 on my Mi5 phone, the notification LED light stopped working for some of the key apps. Such as calendar, Gmail and WhatsApp.
I could not find any way to resolve this, and saw other reports of it as well for others running Oreo-based versions of MIUI.
Anyway, last night I received an OTA update to version 9.6.2.0 and am very relieved to say that the notification LED is working again.