SUMMARY

LUKS partition is set not to automount on login and not to automount on attach in Settings -> Removable Storage -> Removable Devices.

There, selected are checkboxes 
"Enable automatic mounting of removable media"
"Mount all removable media at login"
"Automatically mount removable media when attached"

But in "Device Overrides", all automatic mounting is unchecked.

In file .config/kded_device_automounterrc, all is correct about my LUKS partition. Indeed, no automount attempt takes place.

But then I connect a USB flash drive to the computer and reboot.

Now, immediately after login, windows pop up and ask me to enter three passwords (of KWallet, partition, root) to mount the LUKS partition. But I have set the partition not to automount in "Device Overrides"! 

Now, just because I connected the USB flash drive, I have to cancel in three windows in order to proceed, every time I log in.

So, it looks like the "Device Override" section doesn't override the mounting settings per device.


I looked into and found some possible reasons for automounting attempt: the device files in kded_device_automounterrc have changed because of the flash drive attached during boot: /dev/sdc2 became /dev/sdd2. 
This probably caused the KDE to neglect previously established rules on automounting of /dev/sdc2 (but this is a hypothesis only).


Is it possible to refer to partitions via their UUID instead of device names? This should make obeying the rules more persistent. Many settings in Linux, for example in /etc/fstab, use this method to maintain persistence when dealing with partitions.

Or is there other solution to prevent a certain LUKS partition from automounting in all cases?

Plasma-desktop version is 5.12.8 and plasma-framework version is 5.55.0.




STEPS TO REPRODUCE

1. Create a LUKS partition. Add no records of it in fstab and crypttab.

2. In Settings, check checkboxes 

"Enable automatic mounting of removable media"
"Mount all removable media at login"
"Automatically mount removable media when attached"

and uncheck "Only automatically mount removable media that has been mounted before"

3. In "Device Overrides", uncheck all boxes.

4. Reboot and observe that automount of the LUKS partition is not happening.

5. Plug in a USB flash drive. Mount it, say, by opening it in Dolphin.

6. Reboot.


OBSERVED RESULT

Observe that KDE attempts to automount the LUKS partition. (But this may depend on naming of devices and USB port used).


EXPECTED RESULT

No automounting attempts shall take place, independent of attached USB flash drive, since in "Device Overrides" the automounting of LUKS partition is unchecked.


SOFTWARE/OS VERSIONS

Linux/KDE Plasma: Opensuse 15.1, updated. Running KDE.

KDE Plasma Version: 5.12.8 (probably)
KDE Frameworks Version: 5.55.0
Qt Version: 5.9.7


ADDITIONAL INFORMATION

This bug is similar to 391706. I observed that bug also sometimes, since I got reports on mounting of mounted partition. However, since a few days the double mounting is gone.