Re: I had the same problem. You must have sound or vibration on notification on for the LEDs to operate
In general, yes, seems like that's the _default_ logic in Android 7 now (why?)
But:
I think it also varies by device.
- Nexus 5x: there is a checkbox in the phone's Settings labeled "pulse the LED" which makes it work without sound or vibration
- Samsung's with Android 7: I believe there is a per-app LED setting in the phone's Settings (which unfortunately defaults to off after the upgrade). My S6 is still at 6.0.1, can't check.