RHEL 7.6 Workstation KDE Console Session Locked Up with No Password Prompt

We have some applications that require to run at the RHEL 7.6 Workstation console using a dedicated non-privileged user account. After sometime of no manual activities (while processing are still going on in the background), the console session will get locked up. When that happens, hitting any key on the console will usually displays a password prompt to allow the session to be continued.

But we have experienced on a few occasions (and on different RLEH workstations too) that no password prompt will appear. The console screen will show a digital clock that updates itself every minute.

The top right corner will display the network, sound and power icons. Selecting any of them will return a small window showing the Sound bar , a greyed-out Ethernet Ethernet connection, and a pause (two small vertical bars) icon.

If we select the Pause option, the console screen will go black and it will pause the entire system.

There does not seem to be any option to resume the user application session as no password prompt appears.

I have tried masking the suspend and hibernate targets:
[root@gerbops3 ~]# systemctl list-unit-files | grep masked
hibernate.target masked
suspend.target masked

Unfortunately that did not help.

Does anyone have any idea what has gone wrong?

I have attached a photo to show the console display.

Many thanks in advance.

Peter