Every so often, while in an active session, the confirmation screen for Logout/Suspend/Reboot/etc. comes up. This happens repeatedly a few times and, after canceling every time, it stops happening for a while, then the behavior repeats. There is no remarkable activity associated with this occurrence. There are no peaks in core temperatures, CPU activity, memory/swap usage and network activity. The following is logged in .xsession-errors when this happens:

powerdevil Suspend session triggered with QMap(("Explicit", QVariant(bool, true))("Type", QVariant(uint, 16)))
ksmserver Shutdown called with confirm 1 type -1 and mode -1

Any ideas re what's going on here or where I should look for possible causes?

Plasma 5.12.7
KDE Framework 5.47.0
QT 5.9.5
Kubuntu 18.04

You run a relatively old Plasma version, I bet this has already been solved if it was a bug.

FWIW: if you use a LTS distribution version you need to ask the distribution as they maintain that particular version, upstream doesn't.