Comment 6 for bug 2011380

OK, so I found the problem.

plasma-discover requires qml-module-org-kde-purpose to be installed. Installing that package restores functionality. And running "apt-cache show plasma-discover | grep qml-module-org-kde-purpose" reveals that qml-module-org-kde-purpose is not listed as a Depends or even a Recommends of plasma-discover.

However, qml-module-org-kde-purpose has kdeconnect as a recommends for reasons I do not understand, which means that installing qml-module-org-kde-purpose will usually pull in kdeconnect, and seeding or depending on qml-module-org-kde-purpose will also result in kdeconnect being pulled in by default.

The ideal solution for Lubuntu would be for kdeconnect to be demoted to a Suggests of qml-module-org-kde-purpose (or removed entirely, I can't see any reason it would be related?), and for qml-module-org-kde-purpose to be added as a Depends of plasma-discover. That would fix everything.