@Brook I tried every steps that shows on internet, didn't solved the issue. I have Note 3/Android 5, don't have this issue only on Note 5/Android 7, the phone I'm using currently have this issue with...

@Brook It happens on both.

I have Samsung Note 5/android 7 with delay notification issue. It only happens when phone is on standby mode(screen off) for more than 3 minutes. I don't have this issue with other apps notifications.