This bug concerns only multi-monitor setups.
If there's no window open or are all minimized then Start11 is detected as a full-screen application.
You can easily check that after startup once on the desktop the Focus Assist is set to "Alarms only" instead of "Off".
Disabling the automatic rule to set to Alarms Only for full screen in Settings will change the behavior.
This is quite eco-unfriendly as effectively idling it's going to be detected as gaming/media playback and advanced power saving features will be disabled.
Also will hide any new notification which is not urgent till something else is open.
Same goes for UserNotification API which will detect Start11 as a full-screen application.