If Plasma is launched under Lightdm, all groups the user was in are hidden. The `groups` command shows only one group, same as username. Normal output of the `groups` command should be "username adm sudo dip plugdev cdrom sambashare", e.g. all that groups I was added in. This bug was introduced after last update to 5.12. If Plasma is launched from SDDM or startx command in TTY, this problem doesn't occur. Also this problem doesn't occur in XFCE, launched from Lightdm.
No part of Plasma meddles with your users groups.