I have many reports from Samsung users about wrong work of sound modes switches in Nights Keeper. This problem is actual not for all models but for some of them. The reason of the problem is that Samsung have broken new standard system API for sound control on some firmwares (both Android 5 and Android 5.1). I didn’t found the reliable way how to detect this broken API. I’m still in research.
But there is work around. If you have problems (and actually only if you have) with whitelist work, sound mode switches, alarms and unexpected sounds from other apps, please, try to select Ringer mode in app settings Experimental -> Sound mode control.
Again DO NOT change this setting if you have no problems with app work on your Samsung device.