[SOLVED] KDE Plasma's Discover Notifier suddenly stopped showing up in System Tray

[Sadi]

I've begun using Sparky KDE recently, and preferred using KDE Plasma's own software notifier instead of Sparky's as I'm more used to it. However, after a while (several weeks?) it suddenly stopped showing up in the system tray.
I've re-installed Discover, I can see it both in "/etc/xdg/autostart" and in "System Activity" (as a running process). I can see nothing contradictory in either "System Settings > Notifications > Discover" or "Configure System Tray... > Entries > Updates": it's "Show when relevant" status, but only shows up in when I switch "Always shown").

I wonder why it's not showing up in system tray when updates are available?
I'm afraid it might be related to some Sparky-specific configuration regarding software update notification that interferes with Discover Notifier, which somehow was suppressed at the beginning, and somehow came back to life again.

[Sadi]

The fact that the same problem exists also in Kali Linux probably means this might be resulting from the current state of KDE Plasma packages in Debian Testing (Bookworm) repositories.
So, until this is resolved upstream, a workaround might be a simple script (e.g. like a backend for the Command Output plasmoid) that periodically runs "apt list --upgradable".