Do not disturb issue? Visual disturbances

[AC Question]

Seem to be having an issue and decided to come here. I have a htc u11 and have set do not disturb to only allow priority calls and alarms. Now if my favourite contacts call I get a notification and the phone rings which is fine. However if someone who is not on my favourite contacts calls me, the phone does not ring but the visual notification comes on my screen. I have however blocked visual disturbances under settings do not disturb when screen is both on and off. Can someone shed some light if this is nougat bug or only specific to the htc u11 please as it's driving me up nuts!.