Seems the initial login to plasma, * all systray applets run (even those that are configured to *not* run automatically) * one cannot configure visible systray applets, right click systray -> system tray settings -> general(section-> extra items all of them appear to be unchecked, and no changes made there do anything. This works as expected on subsequent logins. Reproducible: Always
had multiple confirmations in related downstream bug, https://bugzilla.redhat.com/show_bug.cgi?id=1262600 where the specific circumstance there is initial login is enabling an applet that includes X-KDE-PluginInfo-EnabledByDefault=false in it's .desktop file
Tested a bit more, and if it wasn't obvious before, I've confirmed the single trigger for this is whether a user in question has an existing ~/.config/plasma-org.kde.plasma.desktop-appletsrc (no bug) or not (bug)
I remember this, and I'm pretty sure it's fixed now.