So glad I found this thread! I thought I was going mad.
This problem is on the Note9 as well after the May patch. (Build PPR1.180610.011.N960FXXS2CSDJ)
I've wiped my cache partition and the problem persists.
I'm not going to factory reset the phone.
The problem only seems to occur after I've connected a Bluetooth headset (Sony WH-1000XM3)
After that, (even if I've disconnected the headset) notifications are audible whether the phone is on silent or vibrate.
A reboot resolves the issue, but it'll come back if I connect a headset again.
It seems to be tied to media playback and the media volume.
This desperately needs to get resolved as I now can't take my phone into meetings.
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