In September I helped a friend factory-reset her AT&T-locked US-model S8 (G950U I believe) This proceeded with FRP enabled, meaning she signed in with Google credentials to complete the reset. She's been keeping current with OTA software/security updates all along.
Recently she complained of missing texts from some contacts (but not all!) because she didn't get notifications from the native Samsung SMS app. I walked her thru several steps including a check of app-specific notification settings and a wipe of the cache partition.
Eventually I focused on the fact that a few contacts were generating proper notifications, and realized they were all new contacts she'd added after the reset. Looking at her message history revealed that the "mute" setting (bell symbol with a slash) was set for all her earlier contacts. Fixing these one by one solved the missing notifications problem.
She didn't intentionally mute anyone, and I can't find any user-accessible way to mute the entire address book. AFAICT, the only way to do this is manually, on a per-contact basis. So that suggests that the global muting was done by some system update or change. My guess from the timing is the factory reset, but there was also an October security update. Has anyone else encountered this issue?
I have been grappling with why my text notifications have not been coming through until I open the messages app. The identified problem in this discussion is the same as I now identify. Can't tell you how much havoc this has caused in my life for 3 months. Must have been a pre Xmas gift from the Samsung developers. And, thanks for ruining several threads of my life. Insidious happenings in the last 12 months from Samsung.
Now, to go to each and every one of my old contacts and re-enable notifications for each one...