I am using NOKIA6, After NOV monthly security update, we are facing this issue. Even NOKIA support team doesn't have proper info about the fix and ETA. Try this work around: disable and force stop the default 'messages' app and download any third party sms app ( i am using 'sms organizer' of Microsoft) and make this as a defult message app. Then in the battery setting --> activity manager --> white list your sms app. This is working for me.