Can I please have your baseband version? Does this happen if your device is in safe mode?
@user5ym12JisCi give safe mode a try and please keep us posted on if the symptoms still occur.
I would like to add this issue is also a problem with Note9 also. It is not limited to just text notifications. When using a bluetooth headset(Mono earpiece) I lose all notification sounds from my note. I also used to hear the ringtone in my ear piece and from the note simultaneously. Since the security update on Fri evening,only the phone eliminates the ringtone. Like previous users I am missing important alerts when ever a bluetooth device(headset or speaker is in use) Also happens with wired earbuds. Samsung needs to own up to the screw up and stop blaming outside devices. I would list modle,serial number and such,but I don't feel like ruining my Otterbox phone case,especially since this is not my mistake.
Have you tried going to Settings> Sound & Vibration > Advanced Sound Settings? Look for an option to hear audio on a seperate device.