SUMMARY
KDE (& GNOME) in the last few months/years started rolling out an innovative resource management that makes usage of cgroups (thanks David Edmundson).

I assume that this is what "Applications" tab is using to determine to which program a certain process belongs to.
According to the specifications the app units are of the following format:
* app[-<launcher>]-<ApplicationID>[@<RANDOM>].service
* app[-<launcher>]-<ApplicationID>-<RANDOM>.scope
The interesting bit should be the <ApplicationID>.
Note that, assuming to split unit by the dashes, the app_id is in the last and second to last split.

However, using systemd-cgls, you can notice that there is a third possibility:
* app-<ApplicationID>-autostart.service
Even if it is a ".service", the app_id is in the second to last split, not in the last one.

I assume this is what Plasma System Monitor forgets when displaying 

OBSERVED RESULT
https://i.imgur.com/n6Ysb4T.png

SOFTWARE/OS VERSIONS

KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.81.0
Qt Version: 5.15.2