Preset policies have suddenly started notifying users of quarantined messages

OzOscroft 
The only way I've found to prevent users from being notified about quarantined messages is to disable the "Standard Protection" policy:

Once you do that, your custom policies are now the priority and the other policies go back into effect:

It's a "use at your own risk" scenario, which is unfortunate. I think it will also lower your Microsoft "Secure Score" if that's a metric you track. I'm still doing some reading to see if it affects any other components of the M365 Security system.

I would strongly prefer to be able to use Microsoft's recommended Standard or Secure preset security policies without having to worry that the end users will undermine the whole thing by releasing malicious messages into their mailboxes, but that isn't currently possible. 😞