We are also experiencing that iBeacon monitoring is unreliable under iOS 10
I filed a radar: https://openradar.appspot.com/radar?id=4969538768601088
and there are other ones: https://openradar.appspot.com/27987713
Looks like something changed in iOS to the worse, here is another forum entry that seems to be related: Ranging beacons not working if wifi is enabled