This reverts commit 13057013.
That commit set the cursorVisible: propety to true. However that property only exists in the QtQuickControls 2 TextField, not the QQC1 version. We ported the lock screen to PlasmaComponents3 (which internally uses QQC2) after Plasma 5.18, meaning that in the LTS version, we still have a QQC1 TextField so this property does not exist. Trying to set it causes a runtime error and breaks the lock screen.
This went un-detected because, being a runtime error, it did not cause a build failure and writing an integration test for this kind of thing is difficult. Also apparently no KDE developers are actually using the LTS Plasma version (guilty as charged). At least one of these things is probably going to have to change if we want our Plasma LTS promise to mean something. However that's a discussion for another time of course. :)
BUG: 435233 FIXED-IN: 5.18.7.1