I know lots seem to have the same issuse but is there a solution that works, I mean before Samsung stuffed it up with a update many months ago?
When connected to my cars the ring tone comes through but no notifications at all and yes they use too.
One UI, Android 9 etc.
On the plus side its time to change both phones and it would seem that my very long use of Samsung phones is about to end.
I was having the same issue. Theres a setting in the new OS that mutes notifications by default on the phone when it is connected to a bluetooth device. Go to the Samsung Wearables app and select "Notifications". Once there, you'll see the "Mute Connected Phone" setting. Toggle it off and you'll be all set. I hope this helps. Half a day of googling well spent as far as I'm concerned.