SUMMARY


The KDE store plasmoid is "Fokus" ( https://store.kde.org/p/1308861 )

STEPS TO REPRODUCE
1. Right click on the desktop, "Add Widgets..." -> "Get new Widgets..." -> "Download new plasma widgets" -> look for "Fokus" and click install.
2. Go into Latte Dock Settings -> Layout Editor.
3. Create new "Empty" layout, Apply.
4. Switch to new empty layout.
5. Click "Docks, Panels..." button and then add new empty panel, apply now.
6. Right click the new empty panel -> "Add Widgets..." -> Drag and drop a "System Tray" widget on the panel.
7. Right click on panel -> "Add widgets..." -> drag and drop the "Fokus" widget on the Desktop (not in  a panel)
8. Click the notification icon in System Tray, it now says "Notification Service not available/Notification are currently provided by 'KDE Plasma'" and no longer works.

OBSERVED RESULT

Tray notification service is unavailable.

EXPECTED RESULT

Notification works.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 5.23.2
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2
Graphics Platform: X11

ADDITIONAL INFORMATION

If Fokus is placed on the latte panel, notifications keeps working.
I booted into an Activity without Fokus and notifications worked, it's only when switching to an Activity with Fokus on the desktop that notifications stopped working.