I’ve seen this several times both within and without Lubuntu and every time it has been caused by the packager (especially on Snaps where it is often the case that packagers are less experienced) failing to provide a Desktop Entry, which is a required element of the whole Freedesktop Menu Specification which LXQt (and thus Lubuntu) as well as nearly every other Desktop Environment uses. Even the GUI for git which is maintained by the same folks that maintain git suffers this fate.
If this isn’t the case, I’d be absolutely shocked.
To be clear: this has nothing to do with Ubuntu, Lubuntu, LXQt, Snaps, deb packages, or the method to install such packages.